Charles Ellicott Commentary Deuteronomy 4:2

"Ye shall not add unto the word which I command you, neither shall ye diminish from it, that ye may keep the commandments of Jehovah your God which I command you." — Deuteronomy 4:2 (ASV)

Ye shall not add unto the word. — The word, not “the words.” The word is the substance of the Law. The words in which it is expressed may be more or less. The law of Moses contains in it the germ of all revelation to the very end.