When the Messenger of Allah ﷺ prayed, he would hold his arms away from his sides until the whiteness of his armpits became visible.
حدثنا قتيبة بن سعيد حدثنا بكر بن مضر عن جعفر بن ربيعة عن الأعرج
أن رسول الله ﷺ كان إذا صلى فرج بين يديه حتى يبدو بياض إبطيه
Its isnad is sahih. Jafar bin Rabia bin Shurahbil bin Hasanah Kindi Misri is trustworthy, and his hadith is found in the collections of the entire community of scholars. The hadith was mentioned previously at 22820.