Prayer (Kitab Al-Salat): Details of Commencing Prayer
Dawud :: Book 3 : Hadith 1010
Narrated AbuHurayrah:
When the Prophet (peace_be_upon_him) finished two rak'ahs of an obligatory prayer, a man asked him: Apostle of Allah, has the prayer been shortened, or have you forgotten? he replied: I did not do all that. The people said: Apostle of Allah, you did that. Therefore, he offered another two rak'ahs or prayer and did not make two prostrations due to forgetfulness.