‘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
← Previous Hadith “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 Next Hadith → “The Messenger of Allah (ﷺ) led us in one of the afternoon prayers, and he prayed two Rak’ah, then he said the Salam. Then he stood up and went to a piece of wood in the mosque, and leaned against it. Those who were in a hurry left the mosque, saying that the prayer had been shortened. Among the people were Abu Bakr and ‘Umar, but they dared not say anything. Among the people there was also a man with long hands who was called Dhul- Yadain. He said: ‘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, and then he said the Salam again.”

Sunan Ibn Majah Book 5, Hadith 412