Covenant of salt
The phrase covenant of salt appears twice in the Hebrew bible:
In the Book of Numbers, God's covenant with the Aaronic priesthood is said to be a covenant of salt. In the second book of Chronicles, God's covenant with the Davidic kings of Israel is also described as a covenant of salt. According to the New Oxford Annotated Bible, "of salt" most likely means that the covenant is "a perpetual covenant, because of the use of salt as a preservative".
The commandments regarding grain offerings in the Book of Leviticus state "every offering of your grain offering you shall season with salt; you shall not allow the salt of the covenant of your God to be lacking from your grain offering. With all your offerings you shall offer salt."