Jamieson, Fausset and Brown on 1 Timothy 5:18

18. the scripture--(De 25:4; quoted before in 1Co 9:9).

the ox that treadeth out--Greek, An ox while treading.

The labourer is worthy of his reward--or "hire"; quoted from Lu 10:7, whereas Mt 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 De 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.