Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1159
Abu Huraira reported: The Messenger of Allah (may peace be upon him) said: When any one of you stands up to pray. the devil comes to him and confuses him to that he does not know how much he has prayed. If any one of you h" such an experience he should perform two prostrations while sitting down (in qa'da).