The Messenger of Allah ﷺ came in to see me, and I said to him, "Would you be interested in marrying my sister, the daughter of Abu Sufyan?"He asked, "And do what?"I said, "Marry her."He said, "Would you really like that?"I replied, "I am not your only wife, and the one I would most love to share this good with me is my sister." The hadith continues.
دخل علي رسول الله ﷺ فقلت له: هل لك في أختي بنت أبي سفيان؟ فقال: أفعل ماذا؟ قلت تنكحها. قال: أو تحبين ذلك؟ قلت: لست لك بمخلية وأحب من شركني في الخير أختي… الحديث
Bukhari narrated it in the Book of Marriage (5101) via Zuhri, and Muslim narrated it in the Book of Breastfeeding (1449:15) via Hisham (that is, Ibn Urwa), both of them from Urwa bin Zubayr, from Zaynab bint Umm Salama, from Umm Habiba bint Abu Sufyan, who said, and then she mentioned it, and the wording given is Muslim's.Her statement, "I am not exclusively yours," means, I am not alone with you nor free of a co-wife.