Musnad Abu Ya'la al-Mawsili 115:225

Ubayd Allah narrated to us, Yazid ibn Zuray' narrated to us, Sa'id ibn Abi Arubah narrated to us, from Qatadah, that Anas ibn Malik narrated to them: That the Messenger of Allah (may Allah bless him and grant him peace) would not raise his hands in any of his supplications, or at any of his supplications, except in the prayer for rain (istisqa'). For indeed, he would raise his hands until the whiteness of his armpits could be seen.