Riyad as-Salihin, Book 1, Hadith 256 (In-Book) ⓘ

Abu Hurairah (May Allah be pleased with him) reported: A black woman (or probably a young man) used to clean the mosque. Messenger of Allah missed her (or him) and asked about her (or him). He was told that she (or he) had died. He said, "Why did you not inform me?" (It seemed as if) they (Companions) considered the matter insignificant. Then he 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].

وعنه أن امرأة سوداء كانت تقم المسجد، أو شابا، ففقدها، رسول الله صلى الله عليه وسلم، فسأل عنها أو عنه، فقالوا‏:‏ مات‏.‏ قال‏:‏ ‏"‏ أفلا كنتم آذنتموني‏"‏ فكأنهم صغروا أمرها، أو أمره، فقال‏:‏ ‏"‏ دلوني على قبره‏"‏ فدلوه فصلى عليه، ثم قال‏:‏ ‏"‏ إن هذه القبور مملوءة ظلمة على أهلها، وإن الله تعالى ينورها لهم بصلاتي عليهم‏"‏ ‏(‏‏(‏متفق عليه‏)‏‏)‏‏.‏ (23)