"I said to Abu Sa'eed: 'One of us performs Salat and we do not know how (much) we prayed.' So he said: 'Allah's Messenger (S) said: "When one of you performs Salat and he does now know how (much) he prayed then let him perform two prostrations while he is sitting."
Jami` at-Tirmidhi Book 2, Hadith 249
← Previous Hadith the Prophet (S) led them in Salat he forgot (something) so he performed two prostrations, then the Tashah-hud, then the Salam.

Jami` at-Tirmidhi Book 2, Hadith 248 Next Hadith → Allah's Messenger (S) said: "Indeed the Shaitan comes to one of you in his Salat confusing him until he does now know how much he has prayed. When one of you experiences that then let him perform two prostrations while sitting."

Jami` at-Tirmidhi Book 2, Hadith 250