Musannaf Ibn Abi Shayba 17:1234

Ubayd ibn Sa'id narrated to us, from Shu'bah, from Abu Mu'ammal, from al-Zuhri, from 'Urwah, from 'Aishah, that the Prophet (peace and blessings of Allah be upon him), when he prayed the two rak'ahs of the dawn (prayer), would lie down and place his right hand under his right cheek.