Charles Ellicott Commentary Numbers 18:19

"All the heave-offerings of the holy things, which the children of Israel offer unto Jehovah, have I given thee, and thy sons and thy daughters with thee, as a portion for ever: it is a covenant of salt for ever before Jehovah unto thee and to thy seed with thee." — Numbers 18:19 (ASV)

It is a covenant of salt. — that is, an indissoluble covenant. (See Leviticus 2:13, and Note; also 2 Chronicles 13:5.) Hence the phrases used by the Greeks to denote the breach of a covenant, “Where is the salt?” and “They overstepped the salt.” (Compare Pliny, H. N., xxxi. 41; Cicero, De Div., ii. 16; Virgil, Ecl., viii. 82.)