Musnad Ahmad 30:3412

Yazid narrated to us, Muhammad informed us, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the call to prayer is made, the Shaytan turns his back, passing wind so that he does not hear the call. When the caller finishes, he returns to whisper between one of you and his soul, until he makes him forget his prayer, so that he does not know how much he has prayed. So whoever finds any of that, let him prostrate two prostrations."