From the time of the revolt of Zaid, the Shi’ah were divided into Raafidah and Zaidiyyah, and when he (Zaid) was asked about Abu Bakr and ‘Umar he invoked mercy upon them both, a people rejected him (rafadahu), and he said to them, “You have rejected me (rafadtumoonee)”. So they were called “ar-Raafidah”, due to their rejection of him. And anyone who did not reject him from amongst the Shi’ah was called a “Zaidiyy” due to their ascription to him.

Shaykh ul-Islaam Ibn Taymiyyah
Minhaaj as-Sunnah an-Nabawiyyah of Shaykh ul-Islaam Ibn Taymiyyah 1/35
Translated by Abu `Iyaad Amjad Rafiq

Source: http://www.fatwaislam.com