It is narrated from Abu Hurayrah (may Allah be pleased with him) that a black woman used to serve the mosque, and she died. When he (peace and blessings be upon him) missed her, he asked the Companions about her. He was told that she had died, and he (peace and blessings be upon him) said: "Why did you not inform me?" Then he (peace and blessings be upon him) came to her grave and offered the funeral prayer.
Ibn Abdah added in his narration, saying: Hammad informed us that Thabit (may Allah have mercy on him) said that the Messenger of Allah (peace and blessings be upon him) said: "These graves are filled with darkness for the people of the graves, but Allah, Glorified is He, illuminates them through my prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 7014