John 21:24

This is the disciple who testifies of these things, and who has written these things; and we know that his testimony is true.

John 19:35

And he that saw it has given testimony, and his testimony is true, and he knows that he speaks the truth, that you may believe;

3 John 1:12

A good report is given of Demetrius by all, and by the truth itself: and we also give our testimony, and you know that our testimony is true.

John 15:27

and you also will testify, because you have been with me from the beginning.

1 John 1:1-2

THAT which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon, and our hands have handled concerning the Word of life;

1 John 5:6

This is he that came by water and by blood, Jesus the Christ; not by water only, but by water and by blood: and it is the Spirit that testifies, because the Spirit is truth.

Treasury of Scripture Knowledge

Summary

This is the disciple which testifieth of these things, and wrote these things: and we know that his testimony is true.

Bible References

We know

John 19:35
And he that saw it has given testimony, and his testimony is true, and he knows that he speaks the truth, that you may believe;
1 John 1:1
THAT which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon, and our hands have handled concerning the Word of life;
1 John 5:6
This is he that came by water and by blood, Jesus the Christ; not by water only, but by water and by blood: and it is the Spirit that testifies, because the Spirit is truth.
3 John 1:12
A good report is given of Demetrius by all, and by the truth itself: and we also give our testimony, and you know that our testimony is true.