Musnad Ahmad 30:2232

'Affan narrated to us, Hammad ibn Salamah narrated to us, he said: Suhayl ibn Abi Salih narrated to us, from his father, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you finds movement in his posterior during his prayer, and it becomes doubtful to him whether he has passed wind or not, then he should not depart until he hears a sound or finds an odor."