Historical Rules · 1 May 2012
Rule 702.69a
Official source text from the 1 May 2012 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."