Sahih Bukhari
Virtues and Merits of the Prophet (pbuh) and his Companions
Volume: 004 / Book Number: 056 / Hadith Number: 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.")