The Prophet (ﷺ) said the salam, then he spoke, then he performed two prostrations of forgetfulness.
Sunan an-Nasa'i Book 13, Hadith 151
← Previous Hadith "The Messenger of Allah (ﷺ) used to pray eleven rak'ahs, making it odd (witr) by one between the time when he finished 'Isha and dawn, and he would prostrate for as long as it takes one of you to recite fifty verses before raising his head."

Sunan an-Nasa'i Book 13, Hadith 150 Next Hadith → "The Messenger of Allah (ﷺ) said the salam then he performed two prostrations of forgetfulness while he was still sitting, then he said the salam." He said: He mentioned it in the hadith of Dhul-Yadain.

Sunan an-Nasa'i Book 13, Hadith 152