Hebrews 7:5
And verily they that are of the sons of Levi, who receive the office of the priesthood, have a commandment to take tithes of the people according to the law, that is, of their brethren, though they come out of the loins of Abraham: And verily they that are of the sons of Levi, who receive the office of the priesthood, have a commandment to take tithes of the people according to the law, that is, of their brethren, though they come out of the loins of Abraham:
The verse belongs to the account of Abraham giving tithes to Melchizedek, which establishes tithing as Abraham's duty. It sets forth the Lord's plan for the maintenance of the priests who ministered in the temple. The tribe of Levi was chosen and set apart for the sacred offices pertaining to the temple and the priesthood.
Those set apart to this holy office had a commandment according to the law to take tithes of the people, even of their brethren, and were thus supported by those to whom they ministered spiritual blessings. One tenth of all the increase is claimed by the Lord as His own, so to withhold the tithe is robbery toward Him. Since tithing was a duty for Abraham, it remains a duty for those living under the Melchizedek priesthood, who should not fall short of what he did. This same plan is ordained for those who preach the gospel, that they should live of the gospel, for the laborer is worthy of his reward.