26 U.S. Code § 6514 - Credits or refunds after period of limitation
(a) Credits or refunds after period of limitation
A refund of any portion of an internal revenue tax shall be considered erroneous and a credit of any such portion shall be considered void—
(1) Expiration of period for filing claim
If made after the expiration of the period of limitation for filing claim therefor, unless within such period claim was filed; or
(2) Disallowance of claim and expiration of period for filing suit
In the case of a claim filed within the proper time and disallowed by the Secretary, if the credit or refund was made after the expiration of the period of limitation for filing suit, unless within such period suit was begun by the taxpayer.