Musnad Ahmad 5:684

Husayn ibn Muhammad narrated to us, Isra’il narrated to us, from ‘Ata’ ibn al-Sa’ib, who said: I entered upon Abu ‘Abd al-Rahman al-Sulami, and he had prayed the dawn prayer and was sitting in the mosque. I said: "If you were to go to your bed, it would be more comfortable for you." He said: I heard ‘Ali saying: I heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "Whoever prays the dawn prayer, then sits in his place of prayer, the angels pray for him, and their prayer for him is: 'O Allah, forgive him; O Allah, have mercy upon him.' And whoever waits for the prayer, the angels pray for him, and their prayer for him is: 'O Allah, forgive him; O Allah, have mercy upon him.'"