Musnad Abu Ya'la al-Mawsili 1:86

Abu Bakr ibn Abi Shaybah narrated to us, Zayd ibn al-Hubab narrated to us, from Musa ibn Ubaydah, who said: Hud ibn Ata narrated to me, from Anas ibn Malik, who said: Abu Bakr said: The Messenger of Allah (may Allah bless him and grant him peace) forbade striking those who are praying.