Musnad Ahmad 31:330

'Abd al-Razzaq narrated to us, Ma'mar informed us, from Yahya ibn Abi Kathir, he said: 'Iyad ibn Hilal informed me that he heard Abu Sa'id al-Khudri saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If the devil causes doubt for one of you while he is in his prayer, and he says: 'I have broken my ablution,' then let him say to himself: 'You have lied,' until he hears a sound with his ears or finds a smell with his nose. And if one of you prays and does not know whether he added or subtracted, then let him prostrate two prostrations while he is sitting."