פירוש על דברים 6:9

מזזות ביתך THE DOORPOSTS OF THY HOUSE — The word is written מזזת, to indicate that it is necessary to fix only one מזוזה to a door (cf. Menachot 34a).

מזזות ביתיך ובשעריך, mezzuzot both inside your houses and your gates (courtyards.)