Leviticus 24:7

Pure frankincense, unmixed and uncorrupted, or of the best sort. That it may be on the bread, or to the bread, or for the bread, to wit, to be burnt before the Lord instead of the bread, which could not conveniently be offered to God in that manner. And this was done every time that the bread was changed. For a memorial; for that part which properly belonged to God, whereas the rest belonged to the priest. See Poole on "Leviticus 2:2".