From Sahih Muslim (Hadith 1274):
Alqama narrated It on the authority of 'Abdullah (b. Mas'ud) who said:The Messenger of Allah (ﷺ) said the prayer; (the narrator added): He made some act of omission or commission when he pronounced salutation; it was said to him: Messenger of Allah, is there something new about the prayer? He (the Holy Prophet) said: What is it? They said: You said prayer in such and such away. He (the narrator) said: He (the Holy Prophet) turned his feet and faced the Qibla and performed two prostrations and then pronounced salutations, and then turned his face towards us and said: If there is anything new about prayer (new command from the Lord) I informed you of that. But I am a human being and I forget as you for. get, so when I forget, remind me, and when any one of you is in doubt about his prayer. he should aim at what Is correct. and complete his prayer in that respect and then make two prostrations
Reference: Muslim 1274