26 C.F.R. § 1.663(a)-3
No amount deemed to have been distributed to a beneficiary in a preceding year under section 651 or 661 is included in amounts falling within section 661(a) or 662(a). For example, assume that all of the income of a trust is required to be distributed currently to beneficiary A and both the trust and A report on the calendar year basis. For administrative convenience, the trustee distributes in January and February 1956 a portion of the income of the trust required to be distributed in 1955. The portion of the income for 1955 which was distributed by the trust in 1956 may not be claimed as a deduction by the trust for 1956 since it is deductible by the trust and includible in A's gross income for the taxable year 1955.
26 C.F.R. §1.663(a)-3