Mishneh Torah — Transmission of the Oral Law מסירת תורה שבעל פה, Chapter 25

גַּם יִתְבָּאַר מֵהֶם דְּבָרִים שֶׁגָּזְרוּ חֲכָמִים וּנְבִיאִים שֶׁבְּכָל דּוֹר וָדוֹר, לַעֲשׂוֹת סְיָג לַתּוֹרָה, כְּמוֹ שֶׁשָּׁמְעוּ מִמֹּשֶׁה בְּפֵרוּשׁ "וּשְׁמַרְתֶּם אֶת־מִשְׁמַרְתִּי" (ויקרא יח, ל), שֶׁאָמַר עֲשׂוּ מִשְׁמֶרֶת לְמִשְׁמַרְתִּי.

Also, [the sources mentioned above] relate those matters which were decreed by the sages and prophets in each generation in order to "build a fence around the Torah." We were explicitly taught about [this practice] by Moses, as [implied by Leviticus 18:30]: "And you shall observe My precepts," [which can be interpreted to mean]: "Make safeguards for My precepts."