Where in the Bible does it say that the apostle John was boiled in oil?

There is no information in The Bible or any contemporary source as to what happened to John after the death of Jesus. Many imaginative Christian traditions sprang up in the second and third centuries about the travels and martyrdoms of all the disciples, including John, but there is no proof to support any of these traditions.

A popular tradition about John is that Emperor Domitian ordered him brought as a prisoner to Rome and thrown into a vat of boiling oil. John was unharmed by the boiling oil, and he was eventually exiled under guard in Patmos. Not only is the alleged episode of the boiling oil entirely implausible, the alleged exile proves it to be so. If John was immune to harm, as shown by his escape from the boiling oil, he could scarcely have been restrained by armed guards, who also would have been totally unable to harm him. The tradition links John to Patmos because the Book of Revelation was written by a person called John, and this journey was necessary in order to identify the apostle John with the author of Revelation.

So, not only was John never boiled in oil, the Bible does not even claim that he was.