(23) باب صِفَةِ النَّبِيِّ صلى الله عليه وسلم
Narrated `Abdullah bin Malik bin Buhaina Al-Asdi:
When the Prophet (ﷺ) 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.")
Reference: Sahih al-Bukhari 3564
In-book reference: Book 61, Hadith 74
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 4, Book 56, Hadith 765