Sahih Bukhari
Sahih Bukhari Volume 001, Book 012, Hadith Number 771.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 12. Charecteristics Of Prayer
Narated By 'Abdullah bin Malik bin Buhaina : Whenever the Prophet used to offer prayer he used to keep arms away (from the body) so that the whiteness of his armpits was visible.