Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 1031
Narated By Al-Mughirah ibn Shu'bah : The Prophet (pbuh) said: When an imam stands up at the end of two rak'ahs , if he remembers before standing straight up, he should sit down, but if he stands straight up, he must not sit down, but perform the two prostrations of forgetfulness.