What evidence would you like? It's true the primary sources we have to work from were not written in Cato's lifetime. Their authors --men like Plutarch, Livy, Pliny, and Cicero-- were working from records that have been lost to us. It would be a strange thing to fabricate, though: We know quite a bit about Cato the Elder. He was famous for both his accomplishments and for his many firm opinions. There was more than enough for future historians and statesmen to talk about without inventing a catchphrase, and from reading about his life he does seem the sort eccentric enough and stubborn enough to end his speeches in that way. The Romans did love an orator with style: Cato's brand was built in part around his statements of 'there is only one right way to do things!' so why not advocate for the destruction of Carthage at every opportunity? It was what he believed in, and here we are more than two thousand years later still talking about it. What a politician he must have been to have such message discipline and ownership of one of the biggest issues of the day!

In the end, it is part of his myth as it was handed down to us. Is it based on the truth, or were all the men writing about him working from the same flawed information that now appears to be independent complimentary biographical information? There's no way to know for certain.