It was narrated from ‘Abdur-Rahman Al-A’raj that Ibn Buhainah told him that the Prophet (ﷺ) stood up in the second Rak’ah of Zuhr and forgot to sit. When he had finished his prayer, and before he said the Salam, he performed the two prostrations for forgetfulness (Sahw) and said the Salam.
Sunan Ibn Majah Book 5, Hadith 405
← Previous Hadith “The Prophet (ﷺ) offered prayer, I think it was the ‘Asr, and in the second Rak’ah he stood up before he sat. Before he said the Salam, he prostrated twice.”

Sunan Ibn Majah Book 5, Hadith 404 Next Hadith → “The Messenger of Allah (ﷺ) said: ‘If anyone of you stands after two Rak’ah, if he has not yet stood up fully, let him sit down again, but if he has stood up fully, then let him not sit down, and let him perform two prostrations for forgetfulness (Sahw).’”

Sunan Ibn Majah Book 5, Hadith 406