What prophecy does Simon give to Ralph?
The prophecy that you are asking about occurs in chapter 7. The hunters are out hunting, and Ralph is on the beach with Simon. Ralph is in a quiet state of contemplation. He has hope for rescue, but as he looks at the ocean, he realizes that rescue is only a dream. There is almost no chance of it actually happening.
". . . one might dream of rescue; but here, faced by the brute obtuseness of the ocean, the miles of division, one was clamped down, one was helpless, one was condemned, . . ."
Simon appears at Ralph's side and whispers into Ralph's ear the following prophecy:
“You’ll get back to where you came from.”
The statement takes Ralph by surprise, and Ralph responds with a bit of a joke. He asks Simon if he has ship in his pocket. Simon takes the comment well, shakes his head, and repeats that Ralph will get back all right. In total, Simon says the prophecy three times. I'm not sure if Golding was going for a Biblical parallel with that, but Simon is often marked as a Christ figure, and three is an important number that frequently surrounds Christ.
One last thought. The prophecy is definitely disconcerting to Ralph, because it is aimed at Ralph. It doesn't say we will get off the island. It's almost as if Simon is telling Ralph that he (Simon) will not be going home for some reason.