John 8:13. The testimony of one.

The Pharisees challenged him, “Here you are, appearing as your own witness; your testimony is not valid.”

On a technical Jewish point of law, the self-testimony of a single witness was not valid. But they make a mistake in who was doing the testifying. This was not a mere man; The "I am" makes a claim of divine authority as does the mention of light. Light is Yahweh in action - Ps 44:3