Rashi on Numbers 3:1/רש"י על במדבר ג׳:א׳

1ואלה תולדת אהרן ומשה AND THESE ARE THE OFFSPRING OF AARON AND MOSES — But it mentions only the sons of Aaron! But they also are called the sons of Moses because he taught them the Torah. This tells us that whoever teaches the Torah to the son of his fellow man Scripture regards it to him as though he had begotten him (Sanhedrin 19b).

2ביום דבר ה׳ את משה IN THE DAY THAT THE LORD SPAKE TO MOSES did these children (these of Aaron) become his (Moses’) children, because then for the first time he taught them what he heard from the Almighty.