Musnad Ahmad 28:143

Hassan narrated to us, Ibn Lahi'ah narrated to us, Huyayy ibn Abdullah narrated to us, from Abu Abd al-Rahman al-Hubuli, from Abdullah ibn Amr: "That the Messenger of Allah, peace and blessings of Allah be upon him, used to, when he had prayed the two rak'ahs of the dawn (prayer), lie down on his right side."