הלכה על קידושין 79:30

The laws of the commandment and its details are elucidated in many scattered places in the Gemara, such that they, may their memory be blessed, warned us about it, not to ponder thoughts of idolatry (i.e. Berakhot 12b); and they, may their memory be blessed, said that it is not only the thought of idolatry that is forbidden, but rather any thought that causes a person to uproot something from the Torah. And Scripture prohibited this explicitly, as it is written (Numbers 15:39), "and you shall not stray after your hearts." And they may their memories be blessed, said (Kiddushin 40a) that for Israel, the Holy One, blessed be He, does not consider a thought of sin to be like the act - except for the thought of idolatry, which is considered for a man to be like the act (see Mishneh Torah, Laws of Foreign Worship and Customs of the Nations 2).