The Wall Street Journal has the story. See Iran Plots Revenge, U.S. Says:
The Iranian message, intercepted in recent days, came from Qasem Soleimani, the head of Revolutionary Guards' Qods Force, and went to Iranian-supported Shiite militia groups in Iraq, according to U.S. officials.
Maybe this explains why Obama suddenly decided to seek Congressional approval instead of acting alone.
But why are we reading about this? If it was a leak, it shouldn't have been leaked. There's a former information source that is now useless. However, if it wasn't a leak but an official pronouncement then one has to wonder why the administration would do that. The Iranian spokesperson can't be right when he says it was meant to provoke Congress into authorizing a strike on Syria. That sort of threat would make most Congress members more reluctant to authorize a strike.
By the way, to the list of ways Obama has transformed the nation, add this one. He has made conspiracy theorists out of all of us.