'Testimony is True' in the Bible

Jesus answered and said to them, "Even if I do testify concerning Myself, My testimony is true; because I know whence I came, and whither I go; but ye know not whence I came, or whither I go.

And he who has seen has testified, and his testimony is true; and he knows that he says what is true, that ye also may believe.

This is the disciple who testifies concerning these things, and wrote these things; and we know that his testimony is true.