Sahih Muslim Book 5, Hadith Number 2334.

Chapter : It is forbidden to pay Zakat to the Messenger of Allah (may peace be upon him) and Banu Hashim and Banu Muttalib.

‘Ubaidullah b. Abu Rafi’, the freed slave of the Messenger of Allah (may peace be upon him), said: When Haruria (the Khwarij) set out and as he was with ‘Ali b. Abu Talib (Allah be pleased with him) they said, “There is no command but that of Allah.” Upon this ‘Ali said: The statement is true but it is intentionally applied (to support) a wrong (cause). The Messenger of Allah (may peace be upon him described their characteristics and I found these characteristics in them. They state the truth with their tongue, but it does not go beyond this part of their bodies (and the narrator pointed towards his throat). The most hateful among the creation of Allah us one black man among them (Khwarij). One of his hand is like the teat of a goat or the nipple of the breast. When ‘Ali b. Abu Talib (Allah be pleased with him) killed them, he said: Search (for his dead body). They searched for him, but they did not find it (his dead body). Upon this he said: Go (and search for him). By Allah, neither I have spoken a lie nor has the lie been spoken to me. ‘Ali said this twice and thrice. They then found him (the dead body) in a rain. They brought (his dead) body till they placed it before him (Hadrat ‘Ali). ‘Ubaidullah said: And, I was present at (that place) when this happened and when ‘Ali said about them. A person narrated to me from Ibn Hanain that he said: I saw that black man.