Musnad Ahmad 32:1244

Sulayman ibn Dawud narrated to us, Shu'bah informed us, from Thabit, who heard Anas say: "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." I mentioned that to Ali ibn Zayd, and he said: "That is only in the prayer for rain (istisqa)." I said: "Did you hear it from Anas?" He said: "Subhan Allah!" I said: "Did you hear it from him?" He said: "Subhan Allah!"