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 ‏.‏

Narrated Ibn 'Abbas: A prostration when reciting Sad is not one of those which are divinely commanded, but I have seen Messenger of Allah (PBUH) prostrate himself. Translation Not Available

Book reference: Sunan Abi Dawud Book 7 Hadith 1409
Web reference: Sunan Abi Dawud Book 7 Hadith 1404