Eusebius: Ecclesiastical History, II. 15.

So, then, through the visit of the divine word to them, the power of Simon was extinguished, and immediately was destroyed along with the man himself. And such a ray of godliness shone forth on the minds of Peter's hearers, that they were not satisfied with the once hearing or with the unwritten teaching of the divine proclamation, but with all manner of entreaties importuned Mark, to whom the Gospel is ascribed, he being the companion of Peter, that he would leave in writing a record of the teaching which had been delivered to them verbally; and did not let the man alone till they prevailed upon him; and so to them we owe the Scripture called the "Gospel by Mark." On learning what had been done, through the revelation of the Spirit, it is said that the apostle was delighted with the enthusiasm of the men, and sanctioned the composition for reading in the Churches. Clemens gives the narrative in the sixth book of the Hypotyposes.