Bismillah Walhamdulillah Was Salaatu Was Salaam ‘ala Rasulillah
Narrated ‘Abdullah bin Malik bin Buhaina Al-Asdi (Radi-Allahu ‘anhu):
When the Prophet (Sallallahu ‘Alaihi Wa Sallam) 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.”)
Bukhari Vol. 4 : No. 765