Please cite the FAR in which that has been made clear.

FAA Order 8360.38a says the equipment must comply with TSO C129. FAA Orders
are incorporated into CFR 14.


When that policy is promulgated in an FAR it will become legally binding.

I think a pilot using a handheld GPS (not complying with TSO C129) under IFR
would be cited with:
91.13 - Careless and Reckless
91.205(d)(2) - appropriate navigation equipment