Leviticus 24:7

And thou shalt put pure frankincense upon each row, the pure natural gum, as a gift from the people, the twelve tribes of Israel being represented by the twelve cakes of the showbread. The frankincense, according to Jewish tradition, was placed beside each heap of showbread in golden censers, that it may be on the bread for a memorial, even an offering made by fire unto the Lord, its sweet odor serving to bring the people into remembrance before the Lord.