"First of all, I didn't set a red line," said Obama. "The world set a red line. The world set a red line when governments representing 98 percent of the world's population said the use of chemical weapons are [inaudble] and passed a treaty forbidding their use, even when countries are engaged in war. Congress set a red line when it ratified that treaty. Congress set a red line when it indicated that in a piece of legislation entitled the Syria Accountability Act that some of the horrendous things happening on the ground there need to be answered for. So, when I said in a press conference that my calculus about what's happening in Syria would be altered by the use of chemical weapons, which the overwhelming consensus of humanity says is wrong, that wasn't something I just kind of made up. I didn't pluck it out of thin air. There's a reason for it."
Below is a video of Obama setting a "red line" in regard to Syria, from over a year ago. And there is another video, a background call in April 2013 with “a White House official” that left no doubt about what President Obama's own officials thought about where the red line came from ( via weeklystandard.com ).