(Prostrating For) Forgetfulness After The Two Prostrations (Rak'ah) Sunan Abi Dawud 1014
Translation
The Prophet (ﷺ) offered the Zuhr prayer and gave the salutation at the end of two rak'ahs. It was said to him, 'Has the prayer been shortened?' So he prayed two rak'ahs, then performed two prostrations.