Historical Rules · 25 August 2017
Rule 702.70a
Official source text from the 25 August 2017 Rules release
Transfigure is an activated ability. “Transfigure [cost]” means “[Cost], Sacrifice this permanent: Search your library for a creature card with the same converted mana cost as this permanent and put it onto the battlefield. Then shuffle your library. Activate this ability only any time you could cast a sorcery.”