Musnad Ahmad 5:192

Yazid narrated to us, Hammad ibn Salamah narrated to us, from Ya'la ibn 'Ata, from Abdullah ibn Yasar, that 'Amr ibn Hurayth visited Al-Hasan ibn 'Ali. 'Ali said to him, "Do you visit Al-Hasan while there is in your soul what is in it?" 'Amr said, "You are not my Lord so that you may turn my heart wherever You wish." 'Ali said, "As for that, it does not prevent us from offering you advice. I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'There is no Muslim who visits his brother except that Allah sends forth for him seventy thousand angels who pray for him at whatever hour of the day it may be, until he reaches the evening, and at whatever hour of the night it may be, until he reaches the morning.'" 'Amr said to him, "What do you say regarding walking with the funeral procession: in front of it or behind it?" 'Ali said, "The virtue of walking behind it over walking in front of it is like the virtue of the obligatory prayer in congregation over the individual prayer." 'Amr said, "But I saw Abu Bakr and 'Umar walking in front of the funeral procession." 'Ali said, "They disliked to cause hardship to the people."