Abu Dawud Book 2, Hadith Number 1024.
Chapter : Not known.
Narated By AbuSa’id al-Khudri : The Prophet (PBUH) said: When one of you prays, and he does not know whether he prayed more or less rak’ahs (than those prescribed by the Shari’ah), he should perform two prostrations while he is sitting. If the devil comes to him, and tells him (suggests him): “You have been defiled,” he should say: “You have told a lie,” except that he feels smell with his nose, or sound with his ears (then his ablution will break). These are the wording; of the tradition reported by Aban.