Abu Dawud Book 2, Hadith Number 1010.

Narated By AbuHurayrah : When the Prophet (PBUH) 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.