My opponent has a werewolf transforming card in play, currently with the human side face up.
I have a sliver in my hand with sliver cycling. I cycle this sliver to find another in my deck. The one in my hand therefore goes into my graveyard. I end my turn after revealing my chosen sliver and placing it in my hand.
My opponent wants to transform his werewolf, claiming I cast no spells.
I payed mana, a card left my hand and entered my graveyard. Was that a spell?