Rambam - Hilchot Beit Habechirah 7:21

The Laws of God's Chosen House

Moving inwards from the outdoor space known as the “Arena of the Alter” where biblical grade restrictions (at least in the opinion of the Rambam) may already have applied, although by scriptural ordination there was never any formal line of sacred scrimmage drawn.

We now move indoors into the Temple Building itself. Additional restrictions linked to purification procedures become indicative of Higher Holiness. Whether this added layer of sanctity is of biblical origin is a part of the lesson itself.