Per Paul in:
2 Timothy 3:8 ESV
Just as Jannes and Jambres opposed Moses, so these men also oppose the truth, men corrupted in mind and disqualified regarding the faith.
Which is cross referenced to:
Exodus 7:11 ESV
Then Pharaoh summoned the wise men and the sorcerers, and they, the magicians of Egypt, also did the same by their secret arts.
Acts 13:8 ESV
But Elymas the magician (for that is the meaning of his name) opposed them, seeking to turn the proconsul away from the faith.
They profess to know God, but they deny him by their works. They are detestable, disobedient, unfit for any good work.
How did Paul know the names of the Magicians since they appear to not be mentioned anywhere else in the bible by name? And how was this cross reference made (how do we really know that he's talking about the same story)?
I'm going to ask this over at Hermeneutics too. However I feel that this is an interesting doctoral question too.