Original Verse:
1st Corinthians Chapter 9 Verse 2If to others I am not an Apostle, at least I am one to you: for the fact that you are Christians is the sign that I am an Apostle.

Reference Verses:
2nd Corinthians Chapter 3 Verse 2-3 [2] You yourselves are our letter, whose writing is in our heart, open for every man's reading and knowledge; [3] For you are clearly a letter of Christ, the fruit of our work, recorded not with ink, but with the Spirit of the living God; not in stone, but in hearts of flesh.
2nd Corinthians Chapter 3 Verse 2You yourselves are our letter, whose writing is in our heart, open for every man's reading and knowledge;
Acts Chapter 1 Verse 24And they made prayers and said, Lord, having knowledge of the hearts of all men, make clear which of these two has been marked out by you,