Anas b. Malik reported that the Messenger of Allah (ﷺ) prayed for rain pointing the back of his hands to the sky.
Sahih Muslim Book 9, Hadith 6
← Previous Hadith I saw the Messenger of Allah (ﷺ) raising his hands (high enough) in supplication (for rain) that the whiteness of his armpits became visible.

Sahih Muslim Book 9, Hadith 5 Next Hadith → 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 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.

Sahih Muslim Book 9, Hadith 7