You are witnesses, and God also, how holy and righteous and blameless was our conduct toward you believers. (1 Thessalonians 2:10 - ESV)
This is not a bragging or boasting session coming from Paul. It is an open and honest offer at true transparency. Paul has invited them to examine his life under a microscope, so to speak, to see if he is living what he is proclaiming. Paul is never a "do what I say, not what I do" sort of preacher. Instead, he is a "let me show you how it is done" sort of living example.