Historical Rules · 18 June 2010
Rule 702.68a
Official source text from the 18 June 2010 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."