كتاب الصلاة
2  
The Book of Prayer
Bulugh al-Maram 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 .‏ 1
Reference : Bulugh al-Maram 341
In-book reference : Book 2, Hadith 341
English translation : Book 2, Hadith 336
Hadith Text
Reference