He,(PBUH) was saddened because he did not pray the funeral prayer over a woman who used to clean the mosque

 that a black woman used to sweep the mosque, and the Messenger of Allah, sallallaahu ‘alayhi wa sallam, noticed that he had not seen her, so he asked about her, and was told, “She has passed away.” The Prophet of Allah, sallallaahu ‘alayhi wa sallam, said:“Why did you not inform me?” Abu Hurayrah said: “It was as if they considered the matter not worth mentioning.” They showed him where her grave was, and he, sallallaahu ‘alayhi wa sallam, prayed (the funeral prayer) upon her, then said: “These graves are filled with darkness for those residing in them, and Allah brightens them by my praying on them.”1