Narrated Abu Huraira:

A black man or a black woman used to sweep the mosque and he or she died. The Prophet peace be upon him 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."

Narrated Anas ibn Malik: 

The Prophet peace be upon him said: The rewards of my people were presented before me, so much so that even the reward for removing a mote by a person from the mosque was presented to me. The sins of my people were also presented before me. I did not find a sin greater than that of a person forgetting the Qur'anic chapter or verse memorised by him.