If you are asking what John said to Dean right before he died, he told Dran that Sam might turn evil because of what the Yellow Eyed Demon dod to Sam as an infant. He said if you can't save him, you'll have to kill him.
In Hunted, Dean finally reveals to Sam that John had told him that he must save Sam and if he can't, Dean must kill him. This revelation has left Dean shaken ever since, but Dean admits that John never expanded upon what he actually met.
