Problem
The earliest statement about the writing of the Gospel of Mark is from Papias, bishop of Hierapolis about AD 130. Papias' writings no longer exist, but about AD 325, Eusebius the church historian quotes what Papias had to say about the writing of the Gospel of Mark. Read Papias' statement in Eusebius, Ecclesiastical History, Book 3, Chapter 39, Section 15 (you'll have to scroll down to section 15). What information does Papias provide about how Mark wrote his Gospel?