Numbers 18:19

A covenant of salt, i.e. a durable and perpetual covenant; so called here and 2 Chronicles 13:5, either because salt is a sign of incorruption, as being of singular use to preserve things from corruption; or because it is confirmed and ratified on their part by salt, which is therefore called the salt of the covenant, for which the priests were obliged to take care that it should never be lacking from any meat-offering, Leviticus 2:13. And this promise or privilege conferred upon the priests is called a covenant because it is given them conditionally, upon condition of their service, and care about the worship of God, and sacrifices, which were commonly accompanied with meat-offerings, and therefore with salt.