Tradition
St. Jerome
De Viris Illustribus (Illustrious Men)
Chapter 43
Theophilus, bishop of Cæsarea in Palestine, the city formerly called Turris Stratonis, in the reign of the emperor Severus wrote, in conjunction with other bishops, a synodical letter of great utility against those who celebrated the passover with the Jews on the fourteenth day of the month.
Source: De Viris Illustribus (Illustrious Men) (New Advent)