Musnad Ahmad 658:136

Zayd ibn al-Hubab narrated to us, al-Husayn—meaning ibn Waqid—narrated to us, Abu Ishaq narrated to us, al-Bara’ ibn ‘Azib narrated to me, he said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to prostrate upon the heels of his palms."