Musnad Abu Ya'la al-Mawsili 117:228

Zuhayr narrated to us, Yazid narrated to us, Shu'bah informed us, from Thabit, from Anas bin Malik, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to raise his hands in supplication until the whiteness of his armpits could be seen.