I've heard in a sermon that there is a story about Apostle John's last days, when he was working in Ephesus.
The story tells that usually, at the Lord's Supper, John was invited to give a little speech about the church life, and his words was almost always: "Little children, I tell you to love one another". When a fellow worker asked him "Brother, I think we've already understood that. You have walked with the Lord, don't you have another teaching from him?" he answered something like "If, one day, you understand that, there is nothing else you need to learn about the church life".
Firstly, I don't think that the very preacher I've heard it from have made up the story, because he is trustworthy and almost always theologically accurate.
Secondly, I don't remember if the words I used was exactly the same from the story.
That being said, someone know a story like that from any source? Maybe from Irenaeus, or Polycarp, or Tertullian or Justin Martyr?