Deuteronomy 11:20 Halakhah

וּכְתַבְתָּ֛ם עַל־מְזוּז֥וֹת בֵּיתֶ֖ךָ וּבִשְׁעָרֶֽיךָ׃

And thou shalt write them upon the door-posts of thy house, and upon thy gates;