Musnad Ahmad 32:575

Sulayman ibn Dawud narrated to us, Abu 'Amir—meaning al-Khazzaz—narrated to us, from Thabit, from Anas, that a black person used to clean the mosque. He died and was buried at night. The Prophet, peace and blessings of Allah be upon him, was informed, so he said: "Go to his grave." They went to his grave, and he said: "Indeed, these graves are filled with darkness for their inhabitants, and indeed Allah illuminates them through my prayer over them." He came to the grave and prayed over him. A man from the Ansar said: "O Messenger of Allah, my brother died and you did not pray over him." He said: "Where is his grave?" He informed him, and the Messenger of Allah, peace and blessings of Allah be upon him, went with the Ansari.