Reference for Ketubot 146:17

במה דברים אמורים שגירשה כשהיא קטנה והחזירה כשהיא קטנה אבל גירשה כשהיא קטנה והחזירה כשהיא גדולה או שהחזירה כשהיא קטנה וגדלה אצלו ומת או חולצת או מתיבמת

When does this apply? When he divorced her while she was a minor and remarried her while she was still a minor; but if he divorced her while she was a minor and remarried her when she was already of age, or remarried her while she was still a minor and she became of age while she was still with him, and then he died, she may either perform halitzah or contract levirate marriage.

Jastrow