John 21:24

This is the disciple testifying of these, and having written this: and we know that his testimony is true.

John 19:35

And he having seen has testified, and his testimony is true: and he knows that he says true, that ye might believe.

3 John 1:12

It has been testified to Demetrius by all, and by the truth itself: and we also testify; and know that our testimony is true.

John 15:27

And ye also testify, for ye are with me from the beginning.

1 John 1:1-2

What was from the beginning, which we have heard, which we have seen with our eyes, which we beheld, and our own hands felt, concerning the word of life;

1 John 5:6

This is he having come by water and blood, Jesus the Christ; not by water alone, but by water and blood. And the Spirit is testifying, for 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 having seen has testified, and his testimony is true: and he knows that he says true, that ye might believe.
1 John 1:1
What was from the beginning, which we have heard, which we have seen with our eyes, which we beheld, and our own hands felt, concerning the word of life;
1 John 5:6
This is he having come by water and blood, Jesus the Christ; not by water alone, but by water and blood. And the Spirit is testifying, for the Spirit is truth.
3 John 1:12
It has been testified to Demetrius by all, and by the truth itself: and we also testify; and know that our testimony is true.