Sunan Abi Dawud, Hadith 1404 ⓘ
Narrated Ibn 'Abbas: A prostration when reciting Sad is not one of those which are divinely commanded, but I have seen Messenger of Allah prostrate himself.
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا وُهَيْبٌ، حَدَّثَنَا أَيُّوبُ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ لَيْسَ { ص } مِنْ عَزَائِمِ السُّجُودِ وَقَدْ رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَسْجُدُ فِيهَا .
also referenced under: Hadith 1409 (DarusSalam) & Book 7, Hadith 1404 (USC-MSA)