Historical Rules · 30 September 2011
Rule 303.4d
Official source text from the 30 September 2011 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.)