Sahih al-Bukhari 3564
صحيح البخاري#3564SahihNarrated `Abdullah bin Malik bin Buhaina Al-Asdi:When the Prophet (ﷺ) prostrated, he used to keep his arms so widely apart that we used to see his armpits. (The sub-narrator, Ibn Bukair said, "The whiteness of his armpits)
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: 61:74