Helaman 9

Helaman 9:37

"And it came to pass that they went and did, even according as Nephi had said unto them. And behold, the words which he had said were true; for according to the words he did deny; and also according to the words he did confess."

Nephi prophesied concerning the murder of the chief judge.  Nephi knew that the chief judge's brother was responsible for the murder and knew how the murderer would reply to the accusation. All Nephi's prophecies came to pass because he truly received this knowledge from God.  When a Prophet of God gives counsel or predicts something, it's a good idea to listen because what they say will happen.