John Calvin Commentary Psalms 81:4

"For it is a statute for Israel, An ordinance of the God of Jacob." — Psalms 81:4 (ASV)

For this is a statute to Israel. To give greater effect to the preceding exhortation, it is taught here that this law or ordinance had been prescribed to God’s ancient people to ratify the everlasting covenant. And as in covenants there is a mutual agreement between the parties, it is declared that this statute was given to Israel, and that God, in making the covenant, reserved this for himself, as a right to which he was justly entitled.