Sahih Bukhari
Sahih Bukhari Volume 001, Book 008, Hadith Number 385u.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 08. Prayers (Salat)
Narated By 'Abdullah bin Malik : Ibn Buhaina, "When the Prophet prayed, he used to separate his arms from his body so widely that the whiteness of his armpits was visible."