26 CFR 301.6514(a)-1: (a)-1 Credits or refunds after period of limitation.

(a) A refund of any portion of any internal revenue tax (or any interest, additional amount, addition to the tax, or assessable penalty) shall be considered erroneous and a credit of any such portion shall be considered void:

(1) If made after the expiration of the period of limitation prescribed by section 6511 for filing claim therefor, unless prior to the expiration of such period claim was filed, or

(2) In the case of a timely claim, if the credit or refund was made after the expiration of the period of limitation prescribed by section 6532(a) for the filing of suit, unless prior to the expiration of such period suit was begun.

(b) For procedure by the United States to recover erroneous refunds, see sections 6532(b) and 7405.