Yahya related to me from Malik from Ibn Shihab from Abu Salamaibn Abdar-Rahman ibn Awf from Abu Hurayra that the Messenger of Allah,may Allah bless him and grant him peace, said, "When you stand inprayer, Shaytan comes to you and confuses you until you do not knowhow much you have prayed. If you find that happening do two sajdasfrom the sitting position."
App reference: Book 4, Hadith 1
USC-MSA web (English) reference: Book 4, Hadith 1
Arabic reference: Book 4, Hadith 223