Gaius, bishop of Rome, in the time of Zephyrinus, that is, in the reign of Antoninus, the son of Severus, delivered a very notable disputation Against Proculus, the follower of Montanus, convicting him of temerity in his defence of the new prophecy, and in the same volume also enumerating only thirteen epistles of Paul, says that the fourteenth, which is now called, To the Hebrews, is not by him, and is not considered among the Romans to the present day as being by the apostle Paul.
Source: De Viris Illustribus (Illustrious Men) (New Advent)