Musnad Abu Ya'la al-Mawsili 3:2

Abu Khaythama narrated to us, Abu Mu'awiyah narrated to us, al-A'mash narrated to us, from al-Hakam, from Abd al-Rahman ibn Abi Layla, who said: Abu Musa came to al-Hasan ibn Ali to visit him. Ali said to him, "Have you come as a visitor or as a gloater?" He said, "No, rather as a visitor." He said, "If you have come as a visitor, I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'When a man visits his Muslim brother, he walks in the harvest of Paradise until he sits. When he sits, mercy covers him. If it is morning, seventy thousand angels pray for him until he reaches the evening, and if it is evening, seventy thousand angels pray for him until he reaches the morning.'"