Rule 700.3c

Objects grouped into piles don’t leave the zone they’re currently in. If cards in a graveyard are split into piles, the order of the graveyard must be maintained. Example: Fact or Fiction reads, “Reveal the top five cards of your library. An opponent separates those cards into two piles. Put one pile into your hand and the other into your graveyard.” While an opponent is separating the revealed cards into piles, they’re still in their owner’s library. They don’t leave the library until they’re put into their owner’s hand or graveyard.