On what evidence do (Saturday) Sabbatarians claim that the Apostle John kept the Sabbath?

While discussing whether the early church worshipped on the Saturday Sabbath a Sabbatarian (though not an SDA) asserted "We know that the Apostle John kept the (Saturday) Sabbath". My question is simple: How do any Sabbatarians "know" that John kept the Saturday Sabbath? What is the evidence?