On page 206 of the Global Village Contemporary Classics online edition of William Golding's novel Lord of the Flies, Simon has a conversation with the severed pig's head and it tells him

You knew, didn’t you? I’m part of you? Close, close, close! I’m the reason why it’s no go? Why things are what they are?

Towards the end of chapter 8, Simon hallucinates and speaks to the severed pig's head, which is mounted on a stake in the middle of his secluded spot in the forest. During his conversation with the Lord of the Flies, the severed pig's head confirms Simon's belief that there is not a tangible beast on the island. The Lord of the Flies assures Simon that the beast is the inherent wickedness inside each boy on the island and is not something that he can kill. With this knowledge, Simon decides to climb the mountain and discovers that the "beast" is actually the decayed corpse of a deceased paratrooper. The exact page of the quote is different in various editions of the novel and below is the link to the online Global Village Contemporary Classics edition of Lord of the Flies.

Because there are many different editions of William Golding's 1954 novel, providing a definitive page number is impossible. For example, the quotation is found on page 143 of the paperback Perigree edition, a division of Penguin Putnam (cover art linked below). For a page number, you'd have to specify the edition.

However, to help with your question, the scene in which the quotation is found is near the end of chapter eight, "Gift for the Darkness." The conversation in which the Beast, or the Lord of the Flies, identifies itself to Simon takes place only in Simon's mind as he enters a state of psychological and physical breakdown with the impaled pig's head as the only "witness." This scene comes right before the beginning of chapter nine, "A View to a Death," in which Simon will perish.