Musnad Abu Ya'la al-Mawsili 160:35

Hadbah and Ibrahim ibn al-Hajjaj narrated to us, they said: Hammad ibn Salamah narrated to us, from Thabit, from Abu Rafi’, from Abu Hurairah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "The servant remains in prayer as long as he is waiting for the prayer. The angels say: 'O Allah, forgive him; O Allah, have mercy upon him,' until he departs or breaks wind."