Hebrews 7:21

but Jesus became a priest with an oath by the One who said to Him: “The Lord has sworn and will not change His mind: ‘You are a priest forever.’”

Note Psalms 110:4

(For those priests were made without an oath; but this with an oath by him that said unto him, The Lord sware and will not repent, Thou art a priest for ever after the order of Melchisedec:)

(for they indeed have been made priests without an oath), but he with an oath by him that says of him, “The Lord swore and will not change his mind, ‘You are a priest forever, according to the order of Melchizedek.’”

(for they indeed have been made priests without an oath; but he with an oath by him that saith of him, The Lord sware and will not repent himself, Thou art a priest for ever);