Incidentally, I've looked and been unable to find it myself.
"I will advocate for and support a Pain-Capable Unborn Child Protection Act to protect unborn children who are capable of feeling pain from abortion." -- Mitt Romney, NRO, June 18, 2011
Which, as I said, is also on his own campaign website:
http://mittromney.com/blogs/mitts-view/2011/06/my-pro-life-pledge