For the scripture saith, Thou shalt not muzzle the ox that treadeth out the corn. — The quotation is from Deuteronomy 25:4.
The idea in the Apostle’s mind, when he quoted the words of Moses, was: If, in the well-known and loved law of Israel, there was a special reminder to God’s people that the very animals that laboured for them were not to be prevented from enjoying the fruits of their labours, surely men who with zeal and earnestness devoted themselves as God’s servants to their fellows, should be treated with all liberality, and even dignified with especial respect and honour.
And, The labourer is worthy of his reward. — It is possible, though hardly likely, that St. Paul, quoting here a well-known saying of the Lord (see St. Luke 10:7), combines a quotation from a Gospel with a quotation from the Book of Deuteronomy, introducing both with the words “For the Scripture saith” — Scripture (graphè) being always applied by St. Paul to the writings of the Old Testament. It is best and safest to understand these words as simply quoted by St. Paul, as one of the well-remembered precious declarations of the Lord Jesus.