Anas bin Malik (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) prayed for rain, and he (peace and blessings be upon him) did like this: he raised his hands and turned the palms of his hands towards the ground, until I saw the whiteness of his (peace and blessings be upon him) armpits. Ali added these words: 'Wa huwa 'ala al-minbar' (while he (peace and blessings be upon him) was on the pulpit).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6447