It's in paragraph 408(k)(8)(B) of the tax code:
https://www.law.cornell.edu/uscode/text/26/408#k_8
Following all of the references, you end up at paragraph 401(c)(2)(A)(vi) which refers to the deduction for one-half of self-employment taxes not to be included in compensation for this purpose:
https://www.law.cornell.edu/uscode/text/26/401#c
