The following is all based on the excellent book, Tawassul – Its Types and Its Rulings by Shaikh Muhammad Naasiruddeen al-Albaanee (rahimahullah), translated by Aboo Talhah Daawood ibn Ronald Burbank (rahimahullah). Shaikh al-Albaanee replies to this doubt (‘Umar requesting Al-Abbaas to pray for rain) of the Qubooriyoon (the worshippers of graves i.e. Sufis and Shia) in detail (19 pages of this book). However, since this would be too long, we’ve just provided point 1 out of a total of 5 that Shaykh Albani made, so if you want to know the rest of the points, you’ll just have to buy the book!
So one of the doubts that those steeped in innovation and misguidance (beautified by Shaytaan) try to use justify the type of tawassul not permitted is the following hadith:
‘Umar ibn al-Khattaab, radiyallaahu ‘anhu, in times of drought used to ask al-‘Abbaas ibn ‘Abdul-Muttalib to pray for rain on their behalf. He himself would say: “O Allaah we used to ask Your Prophet to supplicate on our behalf to You and You would bless us with rain, and now we ask the uncle of our Prophet to supplicate to You on our behalf, so bless us with rain.” He said: “So they would be blessed with rain.”
The words of Shaikh Albanee in reply:
So they understand from this hadeeth that ‘Umar, radiyallaahu ‘anhu used to use the status of al-‘Abbaas, radiyallaahu ‘anhu, as a means of tawassul, and his position before Allaah. As if his tawassul was merely to mention the name of al-‘Abbaas in his supplication and to ask Allaah to grant them rain because of that. Also that the Companions agreed to this, so this, according to their claim, proves what they hold. As for the reason why ‘Umar, radiyallaahu ‘anhu left off tawassul of the Messenger (صلى الله عليه وسلم) and instead used al-‘Abbaas, radiyallaahu ‘anhu for this, then they claim that this was only in order to show the permissibility of tawassul by means of a person of excellence, even though there are those who are more excellent than him.
This idea of theirs is erroneous, and their explanation is rejected due to many reasons, the most important of them being:
1. A very important principle in the Islamic Sharee’ah is that the Sharee’ah texts explain one another, and none of them are to be understood about a certain matter in isolation from the rest of the texts reported about it. So upon this principle, the previous hadeeth about the tawassul of ‘Umar is to be understood in the light of the other established narrations and ahaadeeth reported about tawassul, after they have been gathered and verified. Then both we and the disputants are agreed that there is a verbal omission in the speech of ‘Umar: “We used to do tawassul to You by means of our Prophet…and now we do tawassul to You by means of the uncle of our Prophet,” and this omission in wording must have as its meaning either: ‘We used to do tawassul to You by means of (the status of) our Prophet, but now we do tawassul to You by means of (the status of) the uncle of our Prophet’, in their view, or ‘We used to do tawassul to You by means of (the supplication of) our Prophet, but now we do tawassul to You by means of (the supplication of) the uncle of our Prophet’, in our view. Now one of these two possible meanings must be taken in order to understand clearly what is being said.
Then in order to known which of these two possible meanings is correct we must refer back to the Sunnah in order to see which way of tawassul by means of the Prophet (صلى الله عليه وسلم) was practiced by the noble Companions. We need to look and see whether, when they suffered drought, each of them sat in his house, or somewhere else, or came together, not in the presence of Allaah’s Messenger (صلى الله عليه وسلم) and then supplicated to their Lord saying: ‘O Allaah by Your Prophet Muhammad, and his honour before You, and his status with You, bless us with rain,’ for example, or whether they used to come to the Prophet (صلى الله عليه وسلم) in person and ask him to supplicate to Allaah, the Most High, for them, and then that he would comply with their request and supplicate to His Lord with full humility until they were blessed with rain.
As for the first of these matters then it is not to be found anywhere in the noble Prophetic ahaadeeth, nor in the practice of the noble Companions, radiyallaahu ‘anhum, and there is no way that any of those in opposition to the way of the Pious Predecessors nor the followers of Sufism will be able to bring a proof to establish that their tawassul was to mention the name of the Prophet (صلى الله عليه وسلم) in their supplications and to make requests to Allaah by his right and his status before Allaah. Rather what we find many examples of, and which the books of Sunnah are replete with the second matter. So we find that the way in which the noble Companions of the Prophet (صلى الله عليه وسلم) performed tawassul was that when they had some need, or they wished for some distress to be removed, then they would go to him (صلى الله عليه وسلم) and ask him directly to supplicate to this Lord for them, i.e. they used the supplication of the noble Messenger (صلى الله عليه وسلم) as a means of nearness (tawassul) to Allaah, the Most High, this and this alone. We are also directed to this by the Saying of Allaah, the Blessed and the Most High:
“If they (the hypocrites), when they had been unjust to themselves, had come to you (Muhammad صلى الله عليه وسلم and begged Allaah’s Forgiveness, and the Messenger had begged forgiveness for them: indeed, they would have found Allaah All-Forgiving, Most Merciful.”
So he (صلى الله عليه وسلم) supplicated to His Lord for them, and He, the One free from all imperfections, answered the supplication of His Prophet, and showed mercy to His servants, and showered His Mercy upon them, and gave life to their barren land.
Also from this is the coming of the same bedouin or someone else on the following Jumu’ah to the Prophet (صلى الله عليه وسلم) again whilst he was delivering the Khutbah. He then complained to him about the roads being cut off, and the collapse of the houses, and drowning of cattle, and he asked him to supplicate to His Lord to withhold the rain for them. So he (صلى الله عليه وسلم) did so and His Lord, the Most Majestic, again answered his supplication.
Another example of this is what the noble lady ‘Aishah, radiyallaahu ‘ahnaa, reports, saying: “The people complained to Allaah’s Messenger (صلى الله عليه وسلم) about the lack of rain, so he ordered for a minbar to be placed in the Prayer-ground, and he set a day for the people to come out. She said: So Allaah’s Messenger (صلى الله عليه وسلم) came out when the rim of the sun appeared and he sat upon the minbar and declared Allaah’s greatness, and praised Him, and then he said: You have complained of drought at your abodes and delay of rain at the start of its season, and Allaah has ordered that you supplicate to Him, and has promised that He will answer your supplication…(the hadeeth).”
In the hadeeth it is mentioned that he (صلى الله عليه وسلم) called upon Allaah, the One free from all imperfections, and led the people in Prayer, and that He, the Most High, granted them rain, so that the streams flowed with water, and the people hurried off to their homes, so the Messenger (صلى الله عليه وسلم) laughed such that his molar teeth were visible and he said: I bear witness that Allaah has full power over everything, and that I am the slave of Allaah and His Messenger.
So these ahaadeeth and their like which occurred in the time of the Prophet (صلى الله عليه وسلم) and the time of the noble Companions, radiyallaahu ‘anhum, clarifying the matter leaving no room for argument or debate, that the tawassul by means of the Prophet (صلى الله عليه وسلم) or the righteous which was practiced by the Pious Predecessors wsa that the one seeking tawassul would come to the one whom he wished to use to perform tawassul and would ask him to supplicate to Allaah, the One free of all imperfections, in order to attain what he wished for. So he would consent to this and then Allaah, the One free of all imperfections, would answer his supplication.
 THE MEANING OF TAWASSUL IN THE ARABIC LANGUAGE
This is that the word Tawassul is an original Arabic word occurring in the Qur’aan and Sunnah and in ancient Arabic poetry and prose, and its meaning is: To draw near to what one seeks after and to approach that which one desires.
Ibnul-Atheer said in an-Nihaayah: “Al-Waasil is one who desires or longs for something, and al-Waseelah is nearness and a means, and that by which one is able to approach and draw near to something. Its plural is Wasaail.”
Al-Fayroozabaadee said in al-Qamoos: “‘He performed waseelah towards Allaah, the Most High,’ means: He did an action in order to draw nearer to Him, as a means of approaching Him.”
THE MEANING OF TAWASSUL IN THE QUR’AAN
“O you who believe! Do your duty to Allaah and fear Him. Seek the means of approach (al-waseelah) to Him, and strive hard in His Cause as much as you can. So that you may be successful.”
[Soorah Al-Maaidah (5):35]
“Those whom they call upon (like Jesus the son Mary, Ezra, etc.) desire (for themselves) means of access (al-waseelah) to their Lord (Allaah), as to which of them should be nearest and they (Jesus, Ezra, angels, etc.) hope for His Mercy and fear His Torment. Verily, the Torment of your Lord is something to be afraid of!”
[Soorah Al-Israa (17):57)]
As for the first Ayaah then the imaam of the scholars of tafseer al-Haafidh Ibn Jareer [at-Tabaree], rahimahullaah, said in explanation of it: “O you who affirm whatever Allaah and His Messenger inform you of, and affirm whatever reward He promised and whatever punishment He threatened: “Fear Allaah.” He says: Respond to Allaah by obeying Him regarding whatever he had ordered or forbidden. “and Seek a Means of approach to Him.” He says: And seek to draw near to Him by doing actions which are pleasing to Him.”
[For more information about the meaning of Tawassul refer to the book pg.1-6]
 So from what has preceded you know that prescribed tawassul, that which is proven by the texts of the Book and the Sunnah, and which is proven by the practice of the Pious Predecessors, and upon which there is a concensus (ijmaa’) of the Muslims is:
1. Tawassul by means of the Names of Allaah, the Blessed and the Most High, and His Attributes.
2. Tawassul by means of a righteous action which the person who is supplicating has done.
3. Tawassul by means of the supplication made by a righteous man.
[Taken from p.38 of the book]
 Reported by Bukhaaree and others.
 Translator’s note: i.e. Unspoken words whose meaning is to be understood to comprehend the full meaning of the sentence.
 Reported by Aby Daawood (translation (1/302/1169), and he said: “This is a rare hadeeth and its chain of narration is saheeh,” and it is as he said, and a group of scholars have declared it authentic as I have pointed out in Saheeh Abee Dawood (no.1064).
Note: this book discusses some weak hadiths used by those justifying making dua to the dead in the graves.