Historical Rules · 1 February 2010
Rule 303.4d
Official source text from the 1 February 2010 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.)