Narrated Ibn 'Abbas (RA):

A prostration while reciting Surat Sad (Surah 38) is not one of the 'Azaa'im (those which are always required), but I have seen Allah's Messenger (ﷺ) prostrating while reciting it. [Reported by al-Bukhari].

وَعَنْ اِبْنِ عَبَّاسٍ ‏-رَضِيَ اَللَّهُ عَنْهُمَا‏- قَالَ : { ( ص )‏ لَيْسَتْ مِنْ عَزَائِمِ اَلسُّجُودِ , وَقَدْ رَأَيْتُ رَسُولَ اَللَّهِ ‏- صلى الله عليه وسلم ‏-يَسْجُدُ فِيهَا } رَوَاهُ اَلْبُخَارِيُّ .‏ [1]‏