Musnad Abu Ya'la al-Mawsili 210:31

Zuhayr narrated to us, Abd al-Jabbar al-Khattabi narrated to us, Ubayd Allah ibn Amr narrated to us, from Abd al-Karim, from Nafi, from Ibn Umar, from Hafsa: That the Prophet, peace and blessings of Allah be upon him, when the caller to prayer (mu'adhdhin) would give the call, he would pray two rak'ahs and food would become forbidden (for the fasting person). And he (the mu'adhdhin) would not give the call until the dawn appeared.