Musnad Ahmad 5:652

Yahya ibn Adam narrated to us, Isra'il narrated to us, from 'Ata' ibn al-Sa'ib, from Abu 'Abd al-Rahman, who said: I heard 'Ali saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, when the servant sits in his place of prayer after the prayer, the angels pray for him. And their prayer for him is: 'O Allah, forgive him; O Allah, have mercy upon him.' And if he sits waiting for the prayer, the angels pray for him, and their prayer for him is: 'O Allah, forgive him; O Allah, have mercy upon him.'"