I noticed it first in Galatians 1.20; cross references led me to Romans 1.9 and 9.1. In all three verses, Paul sounds more like a used car salesman than an apostle.
- Gal 1.20: In what I am writing to you, before God, I do not lie!)
- Romans 1.9: For God is my witness, whom I serve with my spirit in the gospel of his Son, that without ceasing I mention you
- Romans 9.1: I am speaking the truth in Christ—I am not lying; my conscience bears me witness in the Holy Spirit— (ESV throughout)
Perhaps this was a rhetorical device. Perhaps we do have reason to doubt Paul?