“You shall not muzzle the ox when he is threshing.” God taught that the working animal gets to eat while it’s working to bring in your harvest. Does that sound like such an important thing that it should be in both the Old and New Testaments? Well, it is.
In the Old Testament, this teaching is right after the instruction to not degrade or humiliate a guilty brother in his punishment (Deut. 25:1-4).
In the New Testament, it's quoted in a lesson about paying those who do spiritual work:
“For it is written in the Law of Moses, “YOU SHALL NOT MUZZLE THE OX WHILE HE IS THRESHING.” God is not concerned about oxen, is He? Or is He speaking altogether for our sake? Yes, for our sake it was written, because the plowman ought to plow in hope, and the thresher to thresh in hope of sharing the crops” (1 Cor. 9:9,10).
We know that how you treat animals is a good indicator of how you’ll treat people. God is somewhat concerned for the animals—after all, not one falls to the ground without His knowledge (Matt. 10:20)—but what He really cares about is people.
He wants you to learn to share the same concern. In this case, the lesson is everyone gets to eat, the plowman, the thresher and the oxen too. The apostle then applied it to teachers as well. But, beyond that, apply the lesson of universal concern universally—love your neighbor.
All of them.