John 21:24
This is the disciple who testifies about these things, and wrote these things. We know that his witness is true.
John 19:35
He who has seen has testified, and his testimony is true. He knows that he tells the truth, that you may believe.
3 John 1:12
Demetrius has the testimony of all, and of the truth itself; yes, we also testify, and you know that our testimony is true.
John 15:27
You will also testify, because you have been with me from the beginning.
1 John 1:1-2
That which was from the beginning, that which we have heard, that which we have seen with our eyes, that which we saw, and our hands touched, concerning the Word of life
1 John 5:6
This is he who came by water and blood, Jesus Christ; not with the water only, but with the water and the blood. It is the Spirit who testifies, because the Spirit is the truth.