Narrated Ibn Abbas: A prostration when reciting Sad is not one of those which are divinely commanded, but I have seen Messenger of Allah صلی اللہ علیہ وسلم prostrate himself.

Narrated Saeed al-Khudri: The Messenger of Allah صلی اللہ علیہ وسلم 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 صلی اللہ علیہ وسلم 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.