مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4318
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdullah said: The Messenger of Allah (ﷺ) said: “I am only human; I forget as you forget. So if any of you is not sure about his prayer, let him see what is most likely to be the case and then complete it on that basis and prostrate twice."
Musnad Ahmad 4348lts isnad is saheeh, Muslim (572)]