Sifrei_mitzvot for Numbers 4:18

אַל־תַּכְרִ֕יתוּ אֶת־שֵׁ֖בֶט מִשְׁפְּחֹ֣ת הַקְּהָתִ֑י מִתּ֖וֹךְ הַלְוִיִּֽם׃

’Cut ye not off the tribe of the families of the Kohathites from among the Levites;