Documentation below from SSA
https://www.ssa.gov/pubs/EN-05-10536.pdf
PDF page 6 - about half way down - that is the 2nd page of their pamphlet and copied below
To determine if you’ll pay higher premiums, Social Security
uses the most recent federal tax return the IRS provides
to us. If you must pay higher premiums, we use a sliding
scale to make the adjustments, based on your modified
adjusted gross income (MAGI). Your MAGI is your total
adjusted gross income and tax-exempt interest income.
and adjusted gross income (AGI) only includes the taxable portion of SS, meaning the non-taxable portion is not part of AGI.
