26 C.F.R. § 1.662(b)-1
In determining the amount includible in the gross income of a beneficiary, the amounts which are determined under section 662(a) and §§ 1.662(a)-1 through 1.662(a)-4 shall have the same character in the hands of the beneficiary as in the hands of the estate or trust. The amounts are treated as consisting of the same proportion of each class of items entering into the computation of distributable net income as the total of each class bears to the total distributable net income of the estate or trust unless the terms of the governing instrument specifically allocate different classes of income to different beneficiaries, or unless local law requires such an allocation. For this purpose, the principles contained in § 1.652(b)-1 shall apply.
26 C.F.R. §1.662(b)-1