Sahih Muslim: Hadith 2063

Anas reported that the Messenger of Allah (ﷺ) was not accustomed to raice his hands in any supplication he made except when praying for rain. (He would then raise [his hands] high enough) that the whiteness of his armpits became visible. 'Abd al-A'la said that (he was in doubt whether it was) the whiteness of his armpit or armpits.