Sahih Bukhari
Actions while Praying
Volume: 002 / Book Number: 022 / Hadith Number: 324
Narrated Abu Huraira:
Allah's Apostle said, "When anyone of you stands for the prayers, Satan comes and puts him in doubts till he forgets how many Rakat he has prayed. So if this happens to anyone of you, he should perform two prostrations of Sahu while sitting.