Sahih Bukhari
Sahih Bukhari Volume 002, Book 022, Hadith Number 324.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 22. Actions While Praying
Narated By 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.