Hadith 1409
Narrated Ibn Abbas: A prostration when reciting Sad is not one of those which are divinely commanded, but I have seen Messenger of Allah ﷺ prostrate himself.
Hadith 1410
Narrated Saeed al-Khudri: The Messenger of Allah ﷺ recited surah Sad on the pulpit. When he reached the place of prostration (in the surah), he descended and prostrated himself and the people prostrated with him. When the next day came, he recited it. When he reached the place of prostration (in the surah), the people became ready for prostration. Thereupon the Messenger of Allah ﷺ said: This is the repentance of a Prophet ; but I saw you being ready for prostration. So he descended and prostrated himself and the people prostrated along with him.