A black woman (or probably a young man) used to clean the mosque. Messenger of Allah (ﷺ) missed her (or him) and asked about her (or him). He was told that she (or he) had died. He (ﷺ) said, "Why did you not inform me?" (It seemed as if) they (Companions) considered the matter insignificant. Then he (ﷺ) said, "Show me her (or his) grave." When it was shown to him, he offered (Janazah-funeral) prayer over it and said, "These graves cover those in them with darkness, and Allah illumines them for the inmates as a result of my supplication for them".

[Al-Bukhari and Muslim].