Sahih Bukhari Volume 001, Book 008, Hadith Number 448.
Narated By Abu Huraira : A black man or a black woman used to sweep the mosque and he or she died. The Prophet asked about her (or him). He was told that she (or he) had died. He said, "Why did you not inform me? Show me his grave (or her grave)." So he went to her (his) grave and offered her (his) funeral prayer."