הלכה על סנהדרין 128:23

That we should not give from our seed to Molekh: That we should not give some of our sons, to pass them in front of an idol that people would make during the time of the giving of the Torah, the name of which was Molekh; as it is stated (Leviticus 18:21), "And from your seed, you shall not give to pass to Molekh" - meaning some of your seed. And the warning is repeated in a different place, as it is written (Deuteronomy 18:10), "There shall not be found in you, one who passes his son or his daughter in the fire." And so would they do - the father would hand him over to the priests for the sake of the abominations (idols), similar to that which is written about fit offerings (Leviticus 15:14), "and he shall give them to the priest." And it is possible that the priests would do a waving or a presentation in front of the Molekh, and they would give him back to the father afterwards. And they would burn a big fire in front of the Molekh and the father would take his son and pass him through the flame of the fire. And so did they say in Talmud Yerushalmi Sanhedrin 7:10, "He is not liable until he gives [him] over to the priests and he takes him and passes him through. But the opinion of the rabbi, Rashi, may his memory be blessed (Rashi on Sanhedrin 64b, s.v. shragah) and Rambam (Mishneh Torah, Laws of Foreign Worship and Customs of the Nations 6:3), may his memory be blessed, is that he would not burn him, but rather the worship was to pass him through alone. And once he passed him as it was their way of passing, he is liable. And the opinion of Ramban, may his memory be blessed, (Ramban on Leviticus 18:21) is that he would pass him through the flame until his soul departs - and his proofs are in the commentary to the Pentateuch that he made.