Sayyiduna Anas (may Allah be pleased with him) narrates: I saw the Messenger of Allah (peace and blessings be upon him) raising his hands until the whiteness of his (peace and blessings be upon him) armpits was seen, that is, in the supplication for rain (istisqa').
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6446