The Jewish Encyclopedia cites Seder Olam Rabbah 2:2 as a rabbinical claim that Rachel and Leah were twins:
Rachel and Leah were twin sisters, fourteen years old when Jacob came to their father's house; consequently they were twenty-one years old at the time of their marriage to Jacob (Seder 'Olam Rabbah ii.).
I've seen this cited in numerous secondary sources as well, but when I go to the Seder Olam directly I find nothing about Leah and Rachel in the English translation.
Can someone clarify? Can this report be found elsewhere in Seder Olam Rabbah (a typo by the J.E. editors)? Or were they altogether mistaken or what?
note: I've been asked by a 'bot to clarify how this question differs from "how old were Rachel and Leah when they married Jacob?" My question seeks the source of the idea that they were twins. The other answer wants to know their ages when they married.
Addendum: Apparently the translation given at the Sefaria cite above is incomplete. Thanks to those who pointed me to the Hebrew text.