Luke 10:7

For the labourer is worthy of his hire. By hire we must understand not money or its equivalent, but food and nourishment. For the preaching of the kingdom of heaven is above price. Hence S. Augustine says on Ps ciii.: What do they receive? They bestow spiritual gifts, they receive carnal; they give gold, they receive that which is worthless. Therefore it is clear that the apostles should live by the gospel, and that their hearers were bound by every law, natural and divine, to support them. They were forbidden then to carry either purse or scrip, because God put it into the hearts of those that attended on their teaching to provide for all their wants. For S. Gregory says (Hom. 17), He who forbids us to carry scrip or purse, ordains that we should live of the gospel. Because it is fitting that we should receive earthly things from those to whom we offer heavenly rewards. And again, Christ shows why He bade His disciples carry neither scrip nor purse, not because these things are unneeded, but in order to teach that it was the duty of those to whom they were sent to supply them. S. Augustine, De Consent. Evang. lib. ii.