I think you mean the “Nationals and Citizens of the United States at Birth” statute.
http://www.law.cornell.edu/uscode/text/8/1401
The section of the statute that pertains to Senator Cruz qualifying as a citizen 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: Provided, That any periods of honorable service in the Armed Forces of the United States, or periods of employment with the United States Government or with an international organization as that term is defined in section 288 of title 22 by such citizen parent, or any periods during which such citizen parent is physically present abroad as the dependent unmarried son or daughter and a member of the household of a person:
Ted Cruz was born in Calgary, Alberta, Canada, December 22, 1970, to a US citizen mother and a Cuban citizen father.
Relevant law in December 1970 is Pub. L. 82-414 § 301(a)(7) (66 Stat. 163, 236)
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 ten years, at least five of which were after attaining the age of fourteen years: Provided, That any periods of honorable service in the Armed Forces of the United States by such citizen parent may be included in computing the physical presence requirements of this paragraph.
Statutes at Large available online:
http://www.gpo.gov/fdsys/search/citation2.result.STATUTE.action?statute.volume=66&statute.pageNumber=163&publication=STATUTE