Musnad Ahmad 4254

It was narrated from ‘Abdullah that The Messenger of Allah (ﷺ) led them in praying five (rak'ahs), then he turned to face them and the people started whispering to one another. They said: O Messenger of Allah, you prayed five. He turned (to face the qiblah) and led them in prostrating twice and said the salam. And he said: "I am only human; I forget as you forget.”