They performed worship by whistling and clapping their hands.
– the explanation –
From the aspects of the Days of Ignorance that the Messenger of Allaah (صلى الله عليه وسلم) opposed them in was their performance of worship to Allaah by means of whistling and clapping. Allaah says:
وَمَا كَانَ صَلاَتُهُمْ عِندَ الْبَيْتِ إِلاَّ مُكَاء وَتَصْدِيَةً فَذُوقُواْ الْعَذَابَ بِمَا كُنتُمْ تَكْفُرُونَ
“And their prayer at the House (Ka’bah) was nothing but whistling and clapping of hands. So taste the torment because of what you used to disbelieve.” [Surah Al-Anfaal:35]
This means: The worship that the polytheists performed to Allaah at the sacred Ka’bah was nothing more than whistling and clapping.
The word mukaa means whistling, while tasdeeyah means clapping with the hands and palms. They would do this while at the House of Allaah and call it prayer, seeking nearness to Allaah through it. This is one of the things the devils among mankind and the jinn beautified to them, in order to deceive them.
Worship is not to be done except according to what Allaah has legislated – so it is dependent upon revelation. A person must not introduce something from his own self or take it from someone else when it is not according to what Allaah has legislated, thus worshipping Allaah by it even though it has no source in the religion.
Here, we can derive the prohibition of these two matters, which are: whistling and clapping. They are forbidden even though one may not intend to be performing worship by them, since there can be found imitation of the polytheists in it.
As for clapping, the Prophet (صلى الله عليه وسلم) only allowed it specifically1 for women at the time of a necessity, such as to notify the Imaam if he has omitted or added something to the prayer. This was due to the fitnah that could occur from their voice if there are men present. And it is not permissible for a man to imitate the disbelievers or to imitate a women by clapping.
1 Abu Hurairah (رضي الله عنه) reported that the Messenger of Allaah (صلى الله عليه وسلم) said: “Saying Subhaanallaaah is for men and clapping is for women.” [Transmitted by Al-Bukhaaree (no. 1203) and Muslim (no. 422/106)] And in the hadeeth of Sahl bin Sa’ad, he reported that the Messenger of Allaah (صلى الله عليه وسلم) said: “Why is it that I see you clapping so much. Whoever is doubtful about something is the prayer, let him SubhanAllaah for if one says it, it will attract attention. Clapping is only for women.” Transmitted by Al-Bukhaaree (no. 684) and Muslim (no. 421)]
Source: An Explanation Aspects of the Days of Ignorance of Imaam & Mujaddid Muhammad bin ‘Abdil-Wahhaab, by Shaikh Saalih bin Fawzaan al-Fawzaan, Al-Ibaanah Book Publishing, First Edition, 2005/1426H, pp.113-114.