Musnad Abu Ya'la al-Mawsili 115:94

Muhammad ibn al-Minhal al-Darir narrated to us, Yazid ibn Zuray' narrated to us, Sa'id narrated to us, from Qatadah, from Anas ibn Malik, who said: The Messenger of Allah (may Allah bless him and grant him peace) would not raise his hands in any supplication except in the prayer for rain (istisqa'), for he would raise his hands until the whiteness of his armpits could be seen.