When the Messenger of Allah ﷺ prostrated, he would hold his arms away from his sides until the whiteness of his armpits could be seen.
حدثنا عبد الرزاق أنبأنا معمر عن منصور عن سالم بن أبي الجعد
كان رسول الله ﷺ إذا سجد جافى حتى يرى بياض إبطيه
Its chain of transmission is sahih. It has already appeared at 12794.