Musnad Ahmad 1153:400

Mu'awiyah ibn 'Amr narrated to us, he said: Za'idah narrated to us, from Ash'ath ibn Abi al-Sha'tha', from Masruq, from 'Aishah, she said: I asked the Prophet, peace and blessings of Allah be upon him, about looking around during prayer. He said: "It is a snatching that the devil snatches from the servant's prayer."