Sahih Bukhari Volume 2, Book 23, Hadith Number 421.

Narated By Abu Huraira : A black person, a male or a female used to clean the Mosque and then died. The Prophet (p.b.u.h) did not know about it. One day the Prophet remembered him and said, “What happened to that person?” The people replied, “O Allah’s Apostle! He died.” He said, “Why did you not inform me?” They said, “His story was so and so (i.e. regarded him as insignificant).” He said, “Show me his grave.” He then went to his grave and offered the funeral prayer.