Musnad Ahmad 658:131

'Affan narrated to us, 'Ubayd Allah ibn Iyad narrated to us, Iyad narrated to us, 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."