Musannaf Ibn Abi Shayba 6:36

Abu Mu’awiyah narrated to us, from al-A’mash, from al-Hakam, from ‘Abd al-Rahman ibn Abi Layla, who said: Abu Musa came to al-Hasan ibn ‘Ali to visit him while he was ill. ‘Ali said to him, “Have you come as a visitor or as a gloater?” He said, “No, rather as a visitor.” ‘Ali said, “If you have come as a visitor, I heard the Messenger of Allah (may Allah bless him and grant him peace) say: ‘Whoever comes to his Muslim brother to visit him 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 evening, and if it is evening, seventy thousand angels pray for him until morning.’”