U.S.C. § 1401
U.S. Code TITLE 8 > CHAPTER 12 > SUBCHAPTER III > Part I >
§ 1401. Nationals and citizens of United States at birth
The following shall be nationals and citizens of the United States at birth:
(g) a person born outside the geographical limits of the United States and its outlying possessions of parents one of whom is an alien, and the other a citizen of the United States who, prior to the birth of such person, was physically present in the United States or its outlying possessions for a period or periods totaling not less than five years, at least two of which were after attaining the age of fourteen years (this applies to his mother).
http://www.law.cornell.edu/uscode/text/8/1401