Narrated 'Abdullah bin Malik bin Buhaina Al-Asdi: When the Prophet prostrated, he used to keep his arms so widely apart that we used to see his armpits. (The subnarrator, Ibn Bukair said, "The whiteness of his armpits.")
— Sahih Bukhari, Volume 4, Book 56: Virtues and Merits of the Prophet (pbuh) and his Companions, Hadith #765