As said above, there are no extant works or fragments of Pythagoras.
Ancient Pythagoreanism comprised sixth-, fifth- and fourth-century thinkers and many of them attributed their ideas to the founder of the school.
Having said that, the source of Simone de Beauvoir seems to be Aristotle's overview of Pythagoreanism; see Met, Book I, 986a :
the so-called Pythagoreans [...] assumed the elements of numbers to be the elements of everything, and the whole universe to be a proportion or number.
The elements of number, according to them, are the Even and the Odd. Of these the former is limited and the latter unlimited; Unity consists of both (since it is both odd and even); number is derived from Unity; and numbers, as we have said, compose the whole sensible universe. Others of this same school hold that there are ten principles, which they enunciate in a series of corresponding pairs:
(1.) Limit and the Unlimited; (2.) Odd and Even; (3.) Unity and Plurality; (4.) Right and Left; (5.) Male and Female; (6.) Rest and Motion; (7.) Straight and Crooked; (8.) Light and Darkness; (9.) Good and Evil; (10.) Square and Oblong.