Sahih al-Bukhari 807
صحيح البخاري#807SahihNarrated `Abdullah bin Malik bin Buhaina:Whenever the Prophet (ﷺ) used to offer prayer he used to keep arms away (from the body) so that the whiteness of his armpits was visible
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: 10:202