In researching an answer to this question, I found this quote:
It is not permissible to remarry your ex-husband unless you have married someone else. -- muslim1, 2014
I've seen this claim made repeatedly and up until now I thought it was correct. However, I found this fatwa:
If her waiting period ended from this second divorce, she is irrevocably divorced, and he cannot take her back except with a new dowry and a new marriage contract. -- IslamWeb
(And also another IslamWeb fatwa regarding khula.)
It does not state the wife marrying someone else is a pre-condition of remarrying his ex-wife---they just get married again. Hence my question:
Question: Is it impermissible to remarry your ex-husband unless you have married someone else?