26 U.S.C. § 4979

(a) General rule In the case of any plan, there is hereby imposed a tax for the taxable year equal to 10 percent of the sum of— (1) any excess contributions under such plan for the plan year ending in such taxable year, and (2) any excess aggregate contributions under the plan for the plan year ending in such taxable year. (b) Liability for tax The tax imposed by subsection (a) shall be paid by the employer. (c) Excess contributions For purposes of this section, the term “excess contributions” has the meaning given such term by sections 401(k)(8)(B), 408(k)(6)(C), and 501(c)(18). (d) Excess aggregate contribution For purposes of this section, the term “excess aggregate contribution” has the meaning given to such term by section 401(m)(6)(B). For purposes of determining excess aggregate contributions under an annuity contract described in section 403(b), such contract shall be treated as a plan described in subsection (e)(1). (e) Plan For purposes of this section, the term “plan” means— (1) a plan described in section 401(a) which includes a trust exempt from tax under section 501(a), (2) any annuity plan described in section 403(a), (3) any annuity contract described in section 403(b), (4) a simplified employee pension of an employer which satisfies the requirements of section 408(k), and (5) a plan described in section 501(c)(18). Such term includes any plan which, at any time, has been determined by the Secretary to be such a plan. (f) No tax where excess distributed within specified period after close of year (1) In general No tax shall be imposed under this section on any excess contribution or excess aggregate contribution, as the case may be, to the extent such contribution (together with any income allocable thereto through the end of the plan year for which the contribution was made) is distributed (or, if forfeitable, is forfeited) before the close of the first 2½ months (6 months in the case of an excess contribution or excess aggregate contribution to an eligible automatic contribution arrangement (as defined in section 414(w)(3))) of the following plan year. (2) Year of inclusion Any amount distributed as provided in paragraph (1) shall be treated as earned and received by the recipient in the recipient’s taxable year in which such distributions were made.