Eusebius: the Same, VI. 14.

And now, as the blessed Presbyter used to say, since the Lord, as the Apostle of the Almighty, was sent to the Hebrews, Paul, as having been sent to the Gentiles, did not subscribe himself apostle of the Hebrews, out of modesty and reverence for the Lord, and because, being the herald and apostle of the Gentiles, his writing to the Hebrews was something over and above [his assigned function.]