Hebrews 7:20
"And inasmuch as not without an oath he was made priest:"
Cross References (ranked by relevance)
Hebrews 5:6
As he saith also in another place, Thou art a priest for ever after the order of Melchisedec.
Numbers 18:7
Therefore thou and thy sons with thee shall keep your priest’s office for everything of the altar, and within the vail; and ye shall serve: I have given your priest’s office unto you as a service of gift: and the stranger that cometh nigh shall be put to death.
Psalm 110:4
The LORD hath sworn, and will not repent, Thou art a priest for ever after the order of Melchizedek.