Musannaf Ibn Abi Shayba 21:287

Hassan ibn Musa narrated to us, Hammad ibn Salamah narrated to us, Bishr ibn Harb narrated to us, from Abu Sa’id al-Khudri, that the Messenger of Allah (may Allah bless him and grant him peace) used to supplicate at Arafat and raise his hands like this, placing their backs towards his face and their palms towards the ground.