When the Messenger of Allah ﷺ prayed, he would hold his arms away from his sides until the whiteness of his armpits became visible.
أخبرنا قتيبة قال حدثنا بكر عن جعفر بن ربيعة عن الأعرج
أن رسول الله ﷺ كان إذا صلى فرج بين يديه حتى يبدو بياض إبطيه
Bukhari transmitted it, Manaqib, chapter: Description of the Prophet ﷺ, no: 3564, and Muslim, Salat, chapter: Moderation in sujud, and placing the palms on the ground, etc., no: 495, from Qutayba, with this wording. It is also in al-Kubra, no: 693.