Historical Rules · 1 October 2009
Rule 303.4c
Official source text from the 1 October 2009 Rules release
An Aura can't enchant itself. If this occurs somehow, the Aura is put into its owner's graveyard. An Aura that's also a creature can't enchant anything. If this occurs somehow, the Aura becomes unattached, then is put into its owner's graveyard. (These are state-based actions. See rule 704.)