(7) باب السَّهْوِ فِي الْفَرْضِ وَالتَّطَوُّعِ
Narrated Abu Huraira:
Allah's Messenger (ﷺ) said, "When anyone of you stands for the prayers, Satan comes and puts him in doubts till he forgets how many rak`at he has prayed. So if this happens to anyone of you, he should perform two prostrations of Sahu while sitting.
Reference: Sahih al-Bukhari 1232
In-book reference: Book 22, Hadith 10
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 22, Hadith 324