Heb Hebrews, chapter Epistle to the Hebrews, chapter 7
This was attested with a shevu'ah (oath); for others who became kohanim were installed in the office of kehunah without a shevu'ah; But the Rebbe, Melech HaMoshiach became a kohen with a shevu'ah (oath) through the One saying to Him, Hashem has made an oath and will not change his mind, You are a kohen forever.7:21 Ps 110:4 Psa 110:4