Muslim

Anas reported: I saw the Messenger of Allah (ﷺ) raising his hands (high enough) in supplication (for rain) that the whiteness of his armpits became visible.

Anas bin Malik (RA) reported that the Apostle of Allah (ﷺ) prayed for rain pointing the back of his hands to the sky.

Anas reported that the Apostle of Allah (ﷺ) was not accustomed to raise 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. 'Abdul A'la said that (he was in doubt whether it was) the whiteness of his armpit or armpits.

This hadith has been narrated by Anas bin Malik (RA) through another chain of transmitters.