Deuteronomy 25:4

Thou shalt not muzzle the ox when he treadeth out the corn. The usual form of threshing in the Orient is that on a threshing-floor out in the open, where the grain was spread out and oxen driven back and forth across the floor to loosen the grain from the hulls by stepping upon the ears. The humane measure of keeping the animals unmuzzled at that time gave them an opportunity to pick up food whenever they became hungry. It seems that this precept was soon understood in the proverbial sense, in which it is also applied by St. Paul, in his admonition to provide properly for the pastors and teachers, 1 Corinthians 9:9; 1 Timothy 5:18.