The Church historian Eusebius quoted a fragment from Papias concerning the the origin and authorship of the Gospel of Mark in Hist. Eccl. (3, 39):
"And the presbyter said this. 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 [of his hearers], but with no intention of giving a regular 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 fictitious into the statements."
This is what is related by Papias regarding Mark.