Mishkat al-Masabih 9 Supplications كتاب الدعوات 28

وَعَنْ أَنَسٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَرْفَعُ يَدَيْهِ فِي الدُّعَاءِ حَتَّى يُرى بياضُ إبطَيْهِ

Anas said that God’s messenger used to raise his hands in supplication so that the whiteness under his armpits was visible.