Historical Rules · 28 April 2017
Rule 702.102d
Official source text from the 28 April 2017 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.