Sahih al-Bukhari 1069
صحيح البخاري#1069SahihNarrated Ibn `Abbas:The prostration of Sa`d is not a compulsory one but I saw the Prophet (ﷺ) prostrating while reciting it
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 17:3