Deuteronomy 33:9

האמר לאביו ולאמו לא ראיתיו WHO SAID TO HIS FATHER AND TO HIS MOTHER, I HAVE NOT SEEN HIM — When they sinned in the matter of the golden calf and I said, “Who is on the Lord’s side, let him come to me” (Exodus 32:26), all the sons of Levi gathered themselves unto me, and I ordered them each to kill his mother’s father, he being an ordinary Israelite, or his brother on the mother’s side, or the son of his daughter whose husband was an ordinary Israelite, and thus did they do. — It is impossible to explain the term “his father” literally, and “his brothers” as being those on his father's side, and similarly to understand “his sons” literally, for really all these are Levites, and of the tribe of Levi no one sinned., as it is said, “[And there gathered unto me) all the tribe of Levi” (Sifrei Devarim 350:1, Yoma 66b). כי שמרו אמרתך BECAUSE THEY KEPT THY WORD, viz., Thou shalt have no other gods, ובריתך ינצרו AND THEY GUARDED THY COVENANT viz., the covenant of circumcision (Sifrei Bamidbar 67 on בהעלתך); For the [ordinary] Israelites who were born in the desert did not circumcise their sons, whereas the Levites were [not only] circumcised themselves, [but they] also circumcised their sons.

הָאֹמֵ֞ר לְאָבִ֤יו וּלְאִמּוֹ֙ לֹ֣א רְאִיתִ֔יו וְאֶת־אֶחָיו֙ לֹ֣א הִכִּ֔יר וְאֶת־בָּנָ֖ו לֹ֣א יָדָ֑ע כִּ֤י שָֽׁמְרוּ֙ אִמְרָתֶ֔ךָ וּבְרִיתְךָ֖ יִנְצֹֽרוּ׃

who said of his father and of his mother, I have not seen him; nor did he acknowledge his brothers, nor knew his own children: for they have observed thy word, and kept thy covenant.