According to Obama:
Iran’s Supreme Leader has issued a fatwa against the development of nuclear weapons.
However, according to a Middle East Media Research Institute report entitled Renewed Iran-West Nuclear Talks – Part II: Tehran Attempts to Deceive U.S. President Obama, Sec'y of State Clinton With Nonexistent Anti-Nuclear Weapons Fatwa By Supreme Leader Khamenei:
However, an investigation by MEMRI reveals that no such fatwa ever existed or was ever published, and that media reports about it are nothing more than a propaganda ruse on the part of the Iranian regime apparatuses – in an attempt to deceive top U.S. administration officials and the others mentioned above.
Did Khamenei issue a fatwa banning development of nuclear weapons?
Here are some news story about this supposed fatwa: