Musnad Abu Ya'la al-Mawsili 210:5

Abu Talib Abd al-Jabbar ibn Asim narrated to us, Ubayd Allah ibn Amr al-Raqqi narrated to us, from Abd al-Karim al-Jazari, from Nafi, from Ibn Umar, from Hafsa bint Umar: That the Messenger of Allah (may Allah bless him and grant him peace), when the caller to prayer (mu'adhdhin) gave the call for the dawn (fajr), would stand and pray two rak'ahs of the dawn. Then he would go out to the mosque and forbid food. He said: And he would not give the call to prayer until it became morning.