From Sahih Muslim (Hadith 4762):
It has been narrated on the authority of Yahya b. Husain who learnt the tradition from his grandmother. Umm Husain. He said':I heard her say: I performed Hajjat-ul-Wada' in the company of the Messenger of Allah (ﷺ). He said a lot of things (on this occasion). Then I heard him say: If a maimed slave is appointed a commander over you the narrator says: I think she said:" a black stave" who leads you according to the Book of Allah, then listen to him and obey him
Reference: Muslim 4762