Albert Barnes Commentary Jeremiah 7:23

"but this thing I commanded them, saying, Hearken unto my voice, and I will be your God, and ye shall be my people; and walk ye in all the way that I command you, that it may be well with you." — Jeremiah 7:23 (ASV)

Obey ... - These words are not found verbatim in the Pentateuch, but are a summary of its principles. Sacrifice is never the final cause of the covenant, but always obedience (Exodus 19:5–6; Leviticus 11:45. Compare Exodus 20:0;Deuteronomy 11:0, in which the moral object of the Mosaic dispensation is most clearly taught). In connection with Jeremiah’s argument, notice that Amos 5:25 (taken in conjunction with Joshua 5:2-7) proves that the ceremonial law was not observed during the 40 years’ wandering in the wilderness. A thing so long in abeyance in the very time of its founder, could not be of primary importance.