"In your own Law it is written that the testimony of two witnesses is true. I am one who testifies for myself; my other witness is the Father, who sent me.”
Note: another "I AM". Jesus is playing with them. He cites their own law back at them. The irony is that the Saducees among them do not believe in the supernatural so "God" is just a cipher for them; they do not actually believe in God as we know him. That is why it is futile to argue with the Church hierarchy; they also do not believe in a personal God.