Eusebius
The Apostle Peter, after he has established the Church in Antioch, is sent to Rome, where he remains bishop of that city, preaching the Gospel for twenty-five years (The Chronicle, Ad An. Dom. 42 [A.D. 303]).
When Peter preached the Word publicly at Rome, and declared the Gospel by the Spirit, many who were present requested that Mark, who had been for a long time his follower and who remembered his sayings, should write down what had been proclaimed. Having composed the Gospel, he gave it to those who had requested it (Ecclesiastical History 6:14:1 [A.D. 325]).
That makes sense. Mark is short on words. Peter was impetuous and might have often spoken in short coppy sentences such as we find in Mark. Interesting aside.
Have you read the book “Four Witnesses?”
Ignatius of Antioch, Justin Martyr, Clement of Rome, Irenaeus of Lyons — great book that ties these early Church Fathers directly with Bible text.