The Sabbath Fast in Rome: John Cassian (ca. 360-ca. 430 A.D.)

How it was brought about that they fast on the Sabbath in the city.

BUT some people in some countries of the West, and especially in the city [Rome], not knowing the reason of this indulgence, think that a dispensation from fasting ought certainly not to be allowed on the Sabbath, because they say that on this day the Apostle Peter fasted before his encounter with Simon [Magus].

The Institutes of John Cassian, bk. 3, ch. 10

(Schaff, Philip, & Wace, Henry (Eds.), A Select Library of Nicene and Post-Nicene Fathers of the Christian Church, Second Series, Edinburgh: T & T Clark, New York: The Christian Literature Company, 1894, vol. 11, p. 218).