From Vol. III. V. 286
Heb. i. 1. “At sundry times and various manners.”
Since the Lord, being the Apostle of the Almighty, was sent to the Hebrews, it was out of modesty that Paul did not subscribe himself apostle of the Hebrews, from reverence for the Lord, and because he was the herald and apostle of the Gentiles, and wrote the Epistle to the Hebrews in addition [to his proper work].
Source: Fragments (New Advent)