Musnad Ahmad 32:924

Yahya narrated to us, Ibn Abi Arubah narrated to us, from Qatadah, that Anas narrated to them, saying: "The Messenger of Allah, may Allah bless him and grant him peace, would not raise his hands in any of his supplications—and Yahya said once: of the supplication—except in the prayer for rain (Istisqa'). For indeed, he would raise his hands until the whiteness of his armpits was seen."