The Holy Zohar, Kedoshim
We have learnt that it is forbidden for man to gaze at the beauty of a woman lest evil thoughts be aroused in him and lead him to something worse. When R. Shimon went through the town, followed by the Companions, if he saw a beautiful woman he used to lower his eyes and say to the Companions, Do not turn. Whoever gazes at the beauty of a woman by day will have lustful thoughts at night, and if these gain the better of him he will transgress the precept, "Ye shall not make to yourselves molten g-ds". And if he has intercourse with his wife while under the influence of those images, the children born from such union are called "molten g-ds". R. Abba said: 'It is forbidden to a man to fix his gaze upon heathen idols and upon gentile women, or to receive benefit from them in any way.'