John 15:26-27 King James Version (KJV)
26 But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me:

27 And ye also shall bear witness, because ye have been with me from the beginning. John was witness of the words of Christ hearing them from Christ. He was with Christ from the beginning.