Saint Peter
Mark

Traditionally, the Gospel of Mark was said to have been written by a person named John Mark, and that this person was an assistant to Peter; hence its content was traditionally seen as the closest to Peter's viewpoint. According to Eusebius' Ecclesiastical History, Papias recorded this belief from John the Presbyter:

Mark having become the interpreter of Peter, wrote down accurately whatsoever he remembered. It was not, however, in exact order that he related the sayings or deeds of Christ. For he neither heard the Lord nor accompanied Him. But afterwards, as I said, he accompanied Peter, who accommodated his instructions to the necessities, but with no intention of giving a normal or chronological narrative of the Lord's sayings. Wherefore Mark made no mistake in thus writing some things as he remembered them. For of one thing he took especial care, not to omit anything he had heard, and not to put anything fictional into the statements.

Clement of Alexandria in the fragments of his work Hypotyposes (A.D. 190) preserved and cited by the historian Eusebius in his Church History (VI, 14: 6) writes that:

As Peter had preached the Word publicly at Rome, and declared the Gospel by the Spirit, many who were present requested that Mark, who had followed him for a long time and remembered his sayings, should write them out. And having composed the Gospel he gave it to those who had requested it.

Also, Irenaeus wrote about this tradition:

After their (Peter and Paul's) passing, Mark also, the disciple and interpreter of Peter, transmitted to us in writing the things preached by Peter.

Based on these quotes, and on the Christian tradition, the information in Mark's gospel about Peter would be based on eyewitness material. The gospel itself is anonymous, and the above passages are the oldest surviving written testimony to its authorship.
length_constraints:number_words[305]