26 CFR §1.901(j)-1

1. (a)Sourcing rule for certain payments and inclusions. Any income paid or accrued through one or more entities is treated as income from sources within a country described in section 901(j)(2) if the income was, without regard to such entities, from sources within that country.
2. (b)Applicability date. This section applies to taxable years that end on or after December 4, 2018.