“The Messenger of Allah (ﷺ) said: ‘If anyone of you is uncertain about his prayer, let him try to do what is correct then let him prostrate twice.’”

Tanafisi said: "This is the basic rule, and no one is able to reject it."
Sunan Ibn Majah Book 5, Hadith 410
← Previous Hadith “The Messenger of Allah (ﷺ) offered prayer, and I am not sure whether he did something extra or omitted something. He asked, and we told him, so he turned to face the Qiblah and prostrated twice, then he said the Salam. Then he turned to face us and said: ‘If any new command has been revealed concerning the prayer, I would certainly have told you. But I am only human and I forget and you forget. If I forget, then remind me. And if anyone of you is uncertain about the prayer, let him do what is closest to what is correct, then complete the prayer, say the Salam and prostrate twice.”

Sunan Ibn Majah Book 5, Hadith 409 Next Hadith → ‘O Messenger of Allah, has the prayer been shortened or did you forget?’ He said: ‘It has not been shortened and I did not forget.’ He said: ‘But you prayed two Rak’ah.’ He said: ‘Is what Dhul-Yadain says true?’ They said: ‘Yes.’ So he went forward and performed two Rak’ah and said the Salam, then he prostrated twice for prostrations of forgetfulness.

Sunan Ibn Majah Book 5, Hadith 411