Musnad Ahmad 30:3601

Sulayman ibn Dawud narrated to us, Al-Muthanna informed us, from Qatadah, from Abu Ayyub, from Abu Hurayrah, that the Prophet, may God bless him and grant him peace, said: "When one of you fights, let him avoid the face, for God, the Almighty and Majestic, created Adam in His image."