In the law of Moses. See Barnes "Lu 24:44".

Thou shalt not muzzle the mouth, etc. To muzzle, means "to bind the mouth; to fasten the mouth to prevent eating or biting." -- Webster. This was done either by passing straps around the mouth, or by placing, as is now sometimes done, a small basket over the mouth, fastened by straps to the horns of the animal, so as to prevent its eating, but not to impede its breathing freely. This was an instance of the humanity of the laws of Moses. The idea is, that the ox should not be prevented from eating when it was in the midst of food; and that as it laboured for its owner, it was entitled to support; and there was a propriety that it should be permitted to partake of the grain which it was threshing.

That treadeth, etc. This was one of the common modes of threshing in the east, as it is with us. See Barnes "Mt 3:12".

The corn. The grain, of any kind; wheat, rye, barley, etc. Maize, to which we apply the word corn, was then unknown. See Barnes "Mt 12:1".

Doth God take care for oxen? Doth God take care for oxen ONLY? Or is not this rather a principle which shows God's care for all that labour, and the humanity and equity of his laws? And if he is so solicitous about the welfare of brutes as to frame an express law in their behalf, is it not to be presumed that the same principle of humanity and equity will run through all his dealings and requirements? The apostle does not mean to deny that God does take care for oxen, for the very law was proof that he did; but he means to ask whether it is to be supposed that God would regard the comfort of oxen and not of men also? whether we are not to suppose that the same principle would apply also to those who labour in the service of God? He uses this passage, therefore, not as originally having reference to men, or to ministers of the gospel, which cannot be; but as establishing a general principle in regard to the equity and humanity of the Divine laws; and as thus showing that the spirit of the law of God would lead to the conclusion that God intended that the labourer everywhere should have a competent support.

{a} "written in the law" De 25:4; 1 Ti 5:18