Maliks Muwatta Book 004, Hadith Number 001.

Section : What to Do if One Forgets in Prayer.

Yahya related to me from Malik from Ibn Shihab from Abu Salama ibn Abdar-Rahman ibn Awf from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, "When you stand in prayer, Shaytan comes to you and confuses you until you do not know how much you have prayed. If you find that happening do two sajdas from the sitting position."