Sunan At-Tirmidhi

Iyad bin Hilal said:""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