Musnad Abu Ya'la al-Mawsili 110:120

Abu al-Rabi' narrated to us, Salam ibn Sulaym narrated to us, from Zayd al-'Ammi, from Abu Nadrah, from Ibn 'Abbas, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) would prostrate, he would become level, such that if water were poured upon his back, it would remain [there].