When David came to the two hundred men who had been too exhausted to follow him, whom he had left behind at the Wadi Besor, they came out to meet David and the men with him. As David approached, he greeted them. But all the greedy and worthless among those who had accompanied David said, “Since they did not accompany us, we will not give them anything from the plunder, except for each man’s wife and children.” But David said:
“You must not do this, my brothers, after what the LORD has given us. The LORD has protected us and delivered into our hands the raiders that came against us. Who could agree with this proposal of yours? Rather, the share of the one who goes down to battle shall be the same as that of the one who remains with the baggage—they share alike.” And from that day forward he made this a law and a custom in Israel, as it still is today. (1 Samuel 30:21-25)
Thus, when Jesus told the parable of the vineyard workers, all of whom were paid the same wage irrespective of how long they worked, he was not introducing a new teaching. Rather, he was reminding them of the long-standing Davidic practice.
Nor was this the first time that Jesus invoked David's example. In Matthew 12 he compared his own disciples to David and his band, who ate of the sacred show bread reserved to the priests. With these examples in mind, it is perhaps all the more fitting that in Matthew 21, the very chapter after the parable of the vineyard workers, the crowds acclaimed Jesus as the "Son of David." Here was one who revived the forgotten teachings of Israel, who called the people to rededicate themselves to the holiness of God's covenant.