It's common to refer to John as the only apostle who didn't die as a martyr. He probably was a martyr, though. See here. His martyrdom is affirmed by Matthew, Mark, and Papias, and it's referred to indirectly by The Martyrdom Of Polycarp and the second-century heretic Heracleon. Concerning Heracleon, see here.
I've written a series of posts on the death of the apostles.