הלכה על קידושין 140:18

And our Rabbis, may their memory be blessed, also said (Berakhot 24a) that it is forbidden to stare even at the hair of a woman that is forbidden to him. And even to hear her voice with the intention to derive pleasure from it is forbidden. And even to stare at the colored clothes of a woman - meaning to say, beautiful clothes, the way of which is to make them from colored fabric - is forbidden, so long as he knows the woman that wears them, since by seeing the garment, he will come to have improper thoughts about her. And they also distanced [us] that we should not ask about the welfare of a married woman at all, and even through her husband (Kiddushin 70b and Tosefot s.v. ein shoelin).