ENOfficial
Anas said that God’s messenger used to raise his hands in supplication so that the whiteness under his armpits was visible.
Source: sunnah.com
Other translations
Arabic
وَعَنْ أَنَسٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَرْفَعُ يَدَيْهِ فِي الدُّعَاءِ حَتَّى يُرى بياضُ إبطَيْهِ
sunnah.com