It is narrated on the authority of Abu Huraira that a dark-complexioned woman (or a youth) used to sweep the mosque. The Messenger of Allah (peace and blessings be upon him) missed her (or him) and inquired about her (or him). The people told him that she (or he) had died. He asked why they did not inform him, and it appears as if they had treated her (or him) or her (or his) affairs as of little account. He (the Holy Prophet) said: Lead me to her (or his) grave. They led him to that place and he said prayer over her (or him) and then remarked: Verily, these graves are full of darkness for their dwellers. Verily, the Mighty and Glorious Allah illuminates them for their occupants by reason of my prayer over them. (Muslim, Bukhari, Mishkaat)
