Musannaf Ibn Abi Shayba 9:1313

Jarir narrated to us, from Asim al-Ahwal, from al-Sha'bi, who said: "Say to the believing men that they should lower their gaze." I said to al-Sha'bi: "A man looks at a woman, and he does not see anything unlawful of her." He said: "You have no right to follow her with your eye."