Thou shalt not muzzle the ox This is a political precept which recommends to us equity and humanity [1] in general; as we have said in expounding the First Epistle to the Corinthians; [2] for, if he forbids us to be unkind to brute animals, how much greater humanity does he demand towards men! The meaning of this statement, therefore, is the same as if it had been said in general terns, that they must not make a wrong use of the labor of others. At the present day, the custom of treading out the corn is unknown in many parts of France, where they thresh the corn with flails. None but the inhabitants of Provence know what is meant by "treading it out." But this has nothing to do with the meaning; for the same thing may be said about ploughing. The laborer is worthy of his hire He does not quote this as a passage of Scripture, but as a proverbial saying, which common sense teaches to all. In like manner, when Christ said the same thing to the Apostles, (Matthew 10:10,) he brought forward nothing else than a statement approved by universal consent. It follows that they are cruel, and have forgotten the claims of equity, who permit cattle to suffer hunger; and incomparably worse are they that act the same part towards men, whose sweat they suck out for their own accommodation. And how intolerable is the ingratitude of those who refuse support to their pastors, to whom they cannot pay an adequate salary!

Footnotes

1 - "Equite et humanite."

2 - See Commentary on the Corinthians, [9]vol. 1, p. 294.

For the Scripture saith - This is adduced as a reason why a church should show all due respect and care for its ministers. The reason is, that as God took care to make provision for the laboring ox, much more should due attention be paid to those who labor for the welfare of the church.
Thou shalt not muzzle the ox - see this passage explained, and its bearing on such an argument shown, in the notes on 1-Corinthians 9:8-10.
And, The labourer is worthy of his reward - This expression is found substantially in Matthew 10:10, and Luke 10:7. It does not occur in so many words in the Old Testament, and yet the apostle adduces it evidently as a quotation from the Scriptures, and as authority in the case. It would seem probable, therefore, that he had seen the Gospel by Matthew or by Luke, and that he quoted this as a part of Scripture, and regarded the Book from which he made the quotation as of the same authority as the Old Testament. If so, then this may be regarded as an attestation of the apostle to the inspiration of the "Gospel" in which it was found.

The Scripture saith, Thou shalt not muzzle the ox - This is a manifest proof that by τιμη, honor, in the preceding verse, the apostle means salary or wages: "Let the elders that rule well be accounted worthy of double honor," a larger salary than any of the official widows mentioned before, for "the laborer is worthy of his hire." The maintenance of every man in the Church should be in proportion to his own labor, and the necessities of his family. He that does no work should have no wages. In the Church of Christ there never can be a sinecure. They who minister at the altar should live by the altar; the ox that treadeth out the corn should not be muzzled; the laborer is worthy of his hire: but the altar should not support him who does not minister at it; if the ox won't tread out the corn, let him go to the common or be muzzled; if the man will not labor, let him have no hire.

In Deuteronomy 25:4
thou shalt not muzzle the ox that treadeth out the corn; See Gill on 1-Corinthians 9:9. See Gill on 1-Corinthians 9:10. The ox, for its strength and labour, is a fit emblem of a Gospel minister; and its treading the corn out of the husk and ear aptly represents the beating out, as it were, of Gospel truths, by the ministers of it, their making the doctrines of the Gospel clear, plain, and evident to the understandings of men; wherefore, as the ox was not muzzled when it trod out the corn, but might freely and largely feed upon it, so such who labour in the preaching of the Gospel ought to have a sufficient and competent maintenance: for which purpose this citation is made, as also the following:
and the labourer is worthy of his reward; which seems to be taken from Luke 10:7 which Gospel was now written, and in the hands of the apostle; who here, by two testimonies, the one from Moses, and the other from Christ, supports the right of the honourable maintenance of the ministers of the Gospel.

the scripture-- (Deuteronomy 25:4; quoted before in 1-Corinthians 9:9).
the ox that treadeth out--Greek, An ox while treading.
The labourer is worthy of his reward--or "hire"; quoted from Luke 10:7, whereas Matthew 10:10 has "his meat," or "food." If Paul extends the phrase, "Scripture saith," to this second clause, as well as to the first, he will be hereby recognizing the Gospel of Luke, his own helper (whence appears the undesigned appositeness of the quotation), as inspired Scripture. This I think the correct view. The Gospel according to Luke was probably in circulation then about eight or nine years. However, it is possible "Scripture saith" applies only to the passage quoted from Deuteronomy 25:4; and then his quotation will be that of a common proverb, quoted also by the Lord, which commends itself to the approval of all, and is approved by the Lord and His apostle.