Sunan At-Tirmidhi

Abdur-Rahman bin Abdullah bin Sa’d Ar-Razi [and he is Ad-Dashtaki] narrated :that his father informed him, that his father – may Allah have mercy upon him – informed him, he said: “I saw a man in Bukhara upon a mule wearing a black Imamah, saying: ‘It was given to me by the Messenger of Allah.’”