Forgetfulness in prayer and the command to prostrate in the prayer

 351. Abu Sa´id Al-Khudri narrated that the Messenger of Allah sallahu alayhi wa sallam said: ´´When one of you is in doubt about his Salat (prayer) and he does not know how many he has prayed, three or four (Rak´at), he should cast aside his doubt and base his prayer on what he is sure of; then perform two prostrations before Taslim (salutations). If he has prayed five Rak´at, they will make his prayer an even number for him, and if he has prayed exactly four, the two prostrations will be humiliation for the devil.

352. Abu Hurairah narrated: The Messenger of Allah sallahu alayhi wa sallam led us in one of the two evening prayers- Zuhr or ´Ars, and said Taslim (salutations) after two ak´ah, then he went to a piece of trunk, which was placed in the Qiblah in the mosque. He leaned on it looking as if he were angry. Abu Bakr and ´Umar were among the people and they were too afriad to speak to him, and the hasty people rushed out (saying): ´´The prayer has been shortened.´´ But among them was a man called Dhul-Yadain, who said: ´´O Messenger of Allah! Has the prayer been shortened or have you forgot?´´ The Prophet sallahu alayhi wa sallam looked at the right and left and said: ´´What is Dhul-Yadain saying?´´ They said: ´´He is right. You performed only two Rak´ah.´´ Then he performed two (more) Rak´ah and said Taslim then he said Takbir and prostrated himself, then he said Takbir and lifted (his head) and then he said Takbir and prostrated himself, then he said Takbir and lifted (his head). He (the narrator) said: It has been reported to my by ´Imran bin Husain that he said: ´´He (then) said Taslim.´´