I searched somewhat, but unfortunately I couldn’t find its reference. Accidentally I myself have heard this Fatwa as well. But I don't know where its reference is. In my opinion, there must be a reason that you cannot find the references of such questions and it could be related to the shamefulness of these Fatwas(rulings).
Actually the tellers or narrators of these kind of fatawas don't like to make public this shamefulness fatwa(s) (ruling(s)), since it could be a negative advertisement for them and would show their rudeness to the people of the world.
Although they will spread it between their followers to reach their profits, but logically they prevent from spreading of this shamefulness ruling in medias.
On the other hand, note that: there should be such Fatwas which make them doing such attacks (suicide attacks often between Shias...), otherwise nobody will doing suicide attack without any goal ...
Meanwhile, the comment of User5072 can be helpful as he said: this is fitwa of noah el ghazi (نوح القاضی). – user5072 Jul 14 '14 at 9:35
Note: I don't mean that this is 100% impossible for you to find its source.