It didn't take me as long to find at as I thought it might:

FAR 61.3e3

(e) Instrument rating. No person may act as pilot in command of a civil
aircraft under IFR or in weather conditions less than the minimums
prescribed for VFR flight unless that person holds:

(1) The appropriate aircraft category, class, type (if required), and
instrument rating on that person's pilot certificate for any airplane,
helicopter, or powered-lift being flown;

(2) An airline transport pilot certificate with the appropriate
aircraft category, class, and type rating (if required) for the
aircraft being flown;

(3) For a glider, a pilot certificate with a glider category rating and
an airplane instrument rating; or

Brian