פירוש על עבודה זרה 48:20

Rabbis dispute whether Jethro’s visit to Moses (in Exodus 18) happened before or after the giving of the Torah (chapters 19-20). For those who hold that it happened after the giving of the Torah, we still need to resolve how Jethro could bring his own animal as an offering. The answer is that Jethro bought the animal from an Israelite.