Sunan Abi Dawud 1409
سنن أبي داود#1409SahihNarrated Ibn 'Abbas:A prostration when reciting Sad is not one of those which are divinely commanded, but I have seen Messenger of Allah (ﷺ) prostrate himself
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Shuaib Al ArnautSahih
Zubair Ali ZaiSahih Bukhari (1069)
In-book reference: 7:9