Sahih al-Bukhari 390
صحيح البخاري#390SahihNarrated 'Abdullah bin Malik: Ibn Buhaina, "When the Prophet (ﷺ) prayed, he used to separate his arms from his body so widely 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: 8:42