The language זה הדבר (this is the matter) does not come anywhere with respect to commandments that apply for all generations. And when it comes in this way with respect to slaughtering outside of the camp⁶³¹ and the heads of the tribes⁶³², it comes for a גזירה שוה (verbal analogy).

Leviticus 17:2 says,דבר אל אהרן ואל בניו ואל כל בני ישראל ואמרת אליהם זה הדבר אשר צוה יהוה לאמר(Speak to Aaron, and to his sons, and to all the children of Israel, and say to them: This is the thing which the LORD has commanded, saying), sayingזה הדברfor “This is the thing”.

Numbers 30:2 says,וידבר משה אל ראשי המטות לבני ישראל לאמר זה הדבר אשר צוה יהוה(And Moses spoke to the heads of the tribes of the children of Israel, saying: This is the thing which the LORD has commanded), sayingזה הדברfor “This is the thing”.

Leviticus 17:7 says,ולא יזבחו עוד את זבחיהם לשעירם אשר הם זנים אחריהם חקת עולם תהיה זאת להם לדרתם(This shall be a statute for ever to them throughout their generations).

The sages make aגזירה שוה (verbal analogy) that since it saidזה הדברin Leviticus 17 pertaining to slaughtering outside of the camp and it applies for all generations, when it saysזה הדברin Numbers 30:2, it must also apply for all generations⁶³³.

footnotes: ⁶³¹ Leviticus 17:2. ⁶³² Numbers 30:2. ⁶³³ Malbim, HaTorah v’HaMitzvah Acharei Mot 88.