Musnad Abu Ya'la al-Mawsili 110:284

Ibn Numayr narrated to us, Abd al-Salam ibn Harb narrated to us, from Yazid al-Dalani, from Qatadah, from Abu al-Aliyah, from Ibn Abbas, who said: I saw the Messenger of Allah (peace and blessings of Allah be upon him) sleeping until he snored or breathed heavily. I said, "O Messenger of Allah, you have slept." He said, "Wudu is not obligatory except upon one who sleeps while lying down, for when he sleeps while lying down, his joints relax."