Musnad Abu Ya'la al-Mawsili 141:2

Amr ibn al-Dahhak narrated to us, my father narrated to us, Musa ibn Ubaydah narrated to us, from Hud ibn Ata, from Anas, from Abu Bakr, who said: The Messenger of Allah (may Allah bless him and grant him peace) forbade striking those who are praying.