Where in the law is that stated?
It’s in 8 USC SECTION 1401 {point “g”
8 USC 1401
“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:”