Musnad Ahmad 30:1250

Abu Bakr al-Hanafi narrated to us, al-Dahhak bin Uthman narrated to us, from Sa'id al-Maqburi, who said: Abu Hurayrah said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you is in prayer, the Shaytan comes and prods him, just as a man prods his riding beast. When he settles down for him, he passes wind between his buttocks to tempt him away from his prayer. So if one of you finds any of that, let him not depart until he hears a sound or perceives a smell about which he has no doubt."