Musar על עבודה זרה 39:19

They further taught: "'you shall keep yourself from every evil thing' (Devarim 23:10) [this teaches] that one should not look intently at a beautiful woman, even if she be unmarried, or at a married woman even if she be ugly" (Avodah Zara 20b).

Regarding "thought" our Sages already mentioned in the beginning of our Beraitha: "'you shall keep yourself from every evil thing' (Devarim 23:10) - From here R. Pinchas b. Yair said that a person should not have [impure] thoughts in his heart, and thus bring himself to have impurity at night" (Ketubot 46a). They further said: "thoughts of sin are worse than the sin itself" (Yoma 29a) and scripture says explicitly: "evil thoughts are an abomination to G-d" (Mishlei 15:26).