by the Supreme Leader of Iran Khamenni issued in that capacity which subsequently has been treated as a Fatwa and has been treated as binding by President Ahmadinejad.
Of course the Supreme Leader can change his mind but as of the present date he has not done so.
Until he does it is the law of Iran. If Iran is full of religious zealots as everyone likes to say well they have to follow this law.
The talking heads, Republican candidates, and Israel PM who claim Iran is threatening to blow Israel to smithereans simply ignore that is not Iran's policy at all, except possibly in retaliation for an Israelli strike.
This mischaracterization is being used to justify an Israelli preemptive strike.