بُلُوغُ الْمَرَامِ
Bulugh al-Maram 341
The Book of Prayer
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]
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].
Sunnah.com reference: Book 2, Hadith 242