Jeremiah 28:9

"'As for the prophet who prophesies of peace, when the word of the prophet comes to pass, the prophet will be known as one whom the Lord has truly sent.'"

(At this time, Jeremiah has found no other prophets that are truly of God. He's thinking the one he speaks to may be the one, but later finds out that God did not send him, and so he's not a prophet of God.)