Musnad Abu Ya'la al-Mawsili 106:16

Narrated to us Abd al-Rahman ibn Salih al-Azdi, narrated to us Hafs ibn Ghiyath, from Hajjaj ibn Artah, from Abu Ishaq, from al-Bara’: That the Messenger of Allah (may Allah bless him and grant him peace) used to place his face between his palms when he prostrated.