Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 22
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 22, with English translation by Maimonides (Rambam).
לִשְׁמֹר בַּיִת זֶה תָּמִיד, שֶׁנֶּאֱמָר: "וְאַתָּה וּבָנֶיךָ אִתָּךְ לִפְנֵי אֹהֶל הָעֵדֻת" (במדבר יח, ב).
To keep watch over this house continuously, as [Numbers 18:2,4] states: "And you and- your descendants before the Tent of Testimony.... [And they shall keep the watch....]"