20 CFR §404.1052

1. (a)We do not include as wages any payment made—
   1. (1)Into a tax-exempt trust or annuity plan by your employer on behalf of you or your beneficiary; or
   2. (2)From a tax-exempt trust or under an annuity plan to, or on behalf of, you or your beneficiary.
2. (b)The trust must be exempt from tax under sections 401 and 501(a) of the Code, and the annuity plan must be a plan described in section 403(a) of the Code when payment is made.
3. (c)The exclusion does not apply to payments to an employee of the trust for work done as an employee of the trust.