As said before, we know that his home was from the beginning a Christian center of Jerusalem. He must have heard many times about Jesus from fellow Christians. Most likely, he had a source of information without an authority.
Towards the end of the 2nd century there was a man named Papias who liked to obtain and transmit information about Jesus in the early days of the Church. Papias tells us that Mark wrote what he heard from the preaching of Peter, the greatest of the apostles. Certainly, Mark stood so close to Peter, and so near to his heart, that Peter called him his own son (1 Peter 5:13).