Historical Rules · 1 June 2014
Rule 702.102d
Official source text from the 1 June 2014 Rules release
As an Aura spell with bestow begins resolving, if its target is illegal, the effect making it an Aura spell ends. It continues resolving as a creature spell and will be put onto the battlefield under the control of the spell’s controller. This is an exception to rule 608.3a.