It's IRS Publication 915 which, when I read it to figure out how to do this when tax season comes, is unfortunately very confusing. A lot of "if/then" scenarios and worksheets to fill out. However, there's some very good information out there about how much of your benefits (in general) are taxable and how to complete the Lump-Sum Election process, which gives you the opportunity to see whether it's beneficial to break the payment into different tax years. If I'm understanding the publication correctly, you will not be asked to file amended returns for prior years.

There was a column that was helpful to me. If you type "Figuring Out the Taxability of Retroactive SSDI Payments site:jfactivist.typepad.com" into your search bar (without the quotes) it should pop right up in your results.