Musnad Abu Ya'la al-Mawsili 110:161

Abu Bakr narrated to us, Abd al-Salam ibn Harb narrated to us, from Yazid ibn Abd al-Rahman, from Qatadah, from Abu al-Aliyah, from Ibn Abbas, that the Prophet (peace be upon him) said: "There is no ablution required for one who sleeps while prostrating, until he lies down. For indeed, when he lies down, his joints relax."