Virtues and Merits of the Prophet (pbuh) and his Companions

Bukhari :: Book 4 :: Volume 56 :: Hadith 765

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 sub-narrator, Ibn Bukair said, "The whiteness of his armpits.")