Muhammad bin Muslim bin Tadras محمد بن مسلم بن تدرس أبو الزبير · The Book on Jana''iz (Funerals)

"There are three times that the Messenger of Allah prohibited us from performing Salat in, burying our dead in: When the sun's rising appears until it has risen up; when the sun is at the zenith until it passes, and when the sun begins its setting, until it has set."

The Prophet said: "Do not sit on the graves nor perform Salat towards them."