Sunnah.com reference : Book 2, Hadith 242English translation : Book 2, Hadith 336Arabic reference : Book 2, Hadith 341
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 .‏  1‏‏1 ‏- صحيح .‏ رواه البخاري ( 1069 )‏ .‏