Iraq and WMD's (stop rolling your eyes!)

A quick question. What is the evidence that people use to say that Saddam Hussein used WMD’s on his own people? I’m not talking about the anecdotal, “well, duh” stuff here. I was wondering if there are any fairly ironclad sources that confirm that the gas was ordered by the Iraqi leadership and came from within Iraq (as opposed to, say, from Iran, with whom the Iraqis have a long-standing grudge?)