Anas reported that the Messenger of Allah (ﷺ) observed prayer on the grave.
Sahih Muslim Book 11, Hadith 92
← Previous Hadith The hadith as narrated by Shaibani has been narrated through another chain of transmitters.

Sahih Muslim Book 11, Hadith 91 Next Hadith → Lead me to her (or his) grave. They led him to that place and he said prayer over her (or him) and then remarked: Verily, these graves are full of darkness for their dwellers. Verily, the Mighty and Glorious Allah illuminates them for their occupants by reason of my prayer over them.

Sahih Muslim Book 11, Hadith 93