Musnad Ahmad 32:1314

'Abd al-Samad narrated to us, Shu'bah narrated to us, Thabit narrated to us, from Anas, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to raise his hands in supplication until the whiteness of his armpits could be seen." He said: So I mentioned that to 'Ali ibn Zayd, and he said: "That is only in the prayer for rain (istisqa')." He said: I said: "Did you hear it from Anas?" He said: "Subhan Allah!"