What is the origin of the story that the Apostle John taught only "love one another"? 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?
It's from Jerome's Commentary on Galatians, 6:10:


  The blessed John the Evangelist lived in Ephesus until extreme old age.  His disciples could barely carry him to church and he could not muster the voice to speak many words.  During individual gatherings he usually said nothing but, "Little children, love one another." The disciples and brothers in attendance, annoyed because they always heard the same words, finally said, "Teacher, why do you always say this?" He replied with a line worthy of John: "Because it is the Lord's commandment and if it alone is kept, it is sufficient."


Andrew Cain, the translator of this edition of Jerome's commentary, suggests that "the source for this story was possibly Hegesippus's Memoirs via either Clement of Alexandria or Origen."  However, none of the works of Hegesippus survive, so this is speculation.