The Messenger of Allah ﷺ used to raise his hands in supplication until the whiteness of his armpits could be seen.
حدثنا أسود بن عامر حدثنا شعبة عن ثابت
كان رسول الله ﷺ يرفع يديه في الدعاء حتى يرى بياض إبطيه
Its isnad is sahih. It has already been mentioned at 13190.