Sahih Muslim Book 4, Hadith Number 1953.

Chapter : Prayer for rain.

Anas reported that the Apostle of Allah (may peace be upon him) 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. ‘Abd al-A’la said that (he was in doubt whether it was) the whiteness of his armpit or armpits.