A prostration when reciting Sad is not one of those which are divinely commanded, but I have seen Messenger of Allah ﷺ prostrate himself. (Using translation from Abū Dāʾūd 1409)
أَنَّهُ قَالَ فِي السُّجُودِ فِي ص «لَيْسَتْ مِنْ عَزَائِمِ السُّجُودِ وَقَدْ رَأَيْتُ رَسُولَ اللَّهِ ﷺ سَجَدَ فِيهَا»