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.")
Reference: Sahih al-Bukhari 3564 In-book : Book 61, Hadith 74 USC-MSA web (English) : Vol. 4, Book 56, Hadith 765 (deprecated)
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