Genesis Rabbah 8:1

Rabbi Yirmiyah ben Elazar said: When the Holy One created the first Adam, God made it androgynous. That's what it means when it says “male and female God created them.” Rabbi Shmuel bar Nachman said: When the Holy One created the first adam, God created it two faced and then (later) sawed it (in two) creating for it two backs, a back here and a back there. They asked him: But what of the verse “and God took one of his ribs (tzela)?” He answered them, [it really means that] "God took one of the flanks (tzela).” The word [tzela] is also used to describe the flank or side of the tabernacle in Exodus 26. [Translation by Rabbi Steve Greenberg]