Musnad Abu Ya'la al-Mawsili 106:54

Ja'far ibn Humayd al-Kufi narrated to us, Ubayd Allah ibn Iyad narrated to us, from his father, from al-Bara' ibn Azib, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When you prostrate, place your palms [on the ground] and raise your elbows."