5:18 the scripture saith. This quotation from Deuteronomy 25:4 was also cited in I Corinthians 9:7-11, specifically verse 9.

5:18 worthy of his reward. This reference is a remarkable testimony to the divine inspiration of the gospel of Luke, with Paul quoting Luke 10:7 as authoritative Scripture. Paul had been accompanied by Luke on some of his earlier missionary voyages. Luke would even be with him in his last days (II Timothy 4:11). Paul probably had frequent contact with his physician throughout his later life. He must have had ample opportunity to read Luke’s gospel, perhaps even helping him with its composition. He realized not only that it was truly a product of the Spirit’s inspiration, but also that these particular words had been spoken by the Lord Jesus, and thus were of special importance in this connection.