Sandman's Quicksand

You must follow all normal timing rules when casting a spell with mayhem. For example, you can't cast a creature card with mayhem you couldn't normally cast during an opponent's turn, even if you discard that card during an opponent's turn.

Mayhem represents an alternative cost that can be paid to cast a spell from your graveyard if you discarded the card with mayhem that turn. You must pay any additional costs the spell has.