[Machine] He said, "The Messenger of Allah ﷺ delivered a sermon to us one day and recited [a verse from the Quran]. When he came to the prostration, he descended and prostrated and we prostrated with him. He recited it (the verse) again and when he reached the prostration, we hastened to prostrate. When he saw us, he said, 'It is the repentance of a Prophet, but I see that you have prepared yourselves for prostration.' So he descended and prostrated and we prostrated with him."  

أَنَّهُ قَالَ خَطَبَنَا رَسُولُ اللَّهِ ﷺ يَوْمًا فَقَرَأَ ص فَلَمَّا مَرَّ بِالسَّجْدَةِ نَزَلَ فَسَجَدَ وَسَجَدْنَا مَعَهُ وَقَرَأَهَا مَرَّةً أُخْرَى فَلَمَّا بَلَغَ السَّجْدَةَ تَيَسَّرْنَا لِلسُّجُودِ فَلَمَّا رَآنَا قَالَ «إِنَّمَا هِيَ تَوْبَةُ نَبِيٍّ وَلَكِنِّي أَرَاكُمْ قَدِ اسْتَعْدَدْتُمْ لِلسُّجُودِ» فَنَزَلَ فَسَجَدَ وَسَجَدْنَا  

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»