Musannaf Ibn Abi Shayba 33:106

Ishaq ibn Mansur narrated to us, he said: Abu Raja narrated to us, from Muhammad ibn Malik, from al-Bara’ who said: We were with the Prophet, may God bless him and grant him peace, at a funeral. When he reached the grave, the Prophet, may God bless him and grant him peace, knelt by the grave. He said: So I turned around and faced him. He said: Then he wept until he moistened the soil. Then he said: "My brothers, for the likes of this, let the workers work. So prepare."