26 U.S.C. §864(b):

TITLE 26 > Subtitle A > CHAPTER 1 > Subchapter N > PART I > Sec. 864.

Sec. 864. - Definitions and special rules

(b) Trade or business within the United States

     For purposes of this part, part II, and chapter 3, the term ''trade or business within the United States''
     includes the performance of personal services within the United States at any time within the taxable
     year, but does not include -

         (1) Performance of personal services for foreign employer

               The performance of personal services -

               (A)  for a nonresident alien individual, foreign partnership, or foreign corporation, not engaged in
                      trade or business within the United States, or

                (B) for an office or place of business maintained in a foreign country or in a possession of the
                      United States by an individual who is a citizen or resident of the United States or by a
                      domestic partnership or a domestic corporation, by a nonresident alien individual
                      temporarily present in the United States for a period or periods not exceeding a
                      total of 90 days during the taxable year   and whose compensation for such
                      services does not exceed in the aggregate $3,000.

         (2) Trading in securities or commodities

              (A) Stocks and securities

                     (i) In general

                        Trading in stocks or securities through a resident broker, commission agent,
                        custodian, or other independent agent.

                     (ii) Trading for taxpayer's own account

                          Trading in stocks or securities for the taxpayer's own account, whether by the
                          taxpayer or his employees or through a resident broker, commission agent,
                          custodian, or other agent, and whether or not any such employee or agent has
                          discretionary authority to make decisions in effecting the transactions. This
                          clause shall not apply in the case of a dealer in stocks or securities.

              (B) Commodities

                    (i) In general

                        Trading in commodities through a resident broker, commission agent, custodian, or other independent agent.

                    (ii) Trading for taxpayer's own account

                         Trading in commodities for the taxpayer's own account, whether by the
                         taxpayer or his employees or through a resident broker, commission agent,
                         custodian, or other agent, and whether or not any such employee or agent
                         has discretionary authority to make decisions in effecting the transactions.
                         This clause shall not apply in the case of a dealer in commodities.

                    (iii) Limitation

                         Clauses (i) and (ii) shall apply only if the commodities are of a kind
                         customarily dealt in on an organized commodity exchange and if the
                         transaction is of a kind customarily consummated at such place.

              (C) Limitation

                       Subparagraphs (A)(i) and (B)(i) shall apply only if, at no time during the taxable
                       year, the taxpayer has an office or other fixed place of business in the United
                       States through which or by the direction of which the transactions in stocks or
                       securities, or in commodities, as the case may be, are effected.