Musnad Ahmad 30:1916

'Affan narrated to us, Hammad bin Zayd narrated to us, Thabit narrated to us, from Abu Rafi', from Abu Hurairah, that a person—a black person—used to sweep the mosque, and he died—or she died—and the Prophet (ﷺ) missed him/her. He said: "What happened to the person who used to sweep the mosque?" It was said to him: "He/she has died." He said: "Why did you not inform me about it?" They said: "It was at night." He said: "Then show me to his/her grave." He said: So he came to the grave and prayed over him/her. Thabit said—at that time, or in another hadith—: "Indeed, these graves are filled with darkness for their inhabitants, and indeed Allah, the Mighty and Majestic, illuminates them through my prayer over them."