When the Messenger of Allah ﷺ prostrated, he would hold his arms away from his sides so that the whiteness of his armpits could be seen by those behind him.
حدثنا وكيع حدثنا جعفر بن برقان عن يزيد بن الأصم
كان رسول الله ﷺ إذا سجد جافى حتى يرى من خلفه بياض إبطيه