Musannaf Ibn Abi Shayba 3:2169

Abu [Abd al-Rahman] Ishaq ibn Mansur narrated to us, he said: Huraym narrated to us, from Yazid ibn Abi Habib, from Abu al-Aswad, from Imran ibn Husayn regarding the verse: "And those who are constant in their prayer," he said: "The one who does not turn [his attention or gaze] during his prayer."