I'm not a tax person, but probably all they need is an amended tax return (1040X) with the sale reported. Grab the instructions for schedule D for the tax year you sold your home, and follow the section for "Sale of Your Home". It says you're only required to report the sale if the gains exceeded the exclusion amount or you received a 1099-S; but I'm guessing the IRS must have received a 1099-S even if you didn't. You might be able to get a copy of that through the IRS Tax Transcript service?

They probably don't need a copy of the Statute, but eh. :)