Why he refused a marriage his own wife proposed
Umm Habibah suggested her sister and gave her reason.
لَوْ لَمْ تَكُنْ رَبِيبَتِي مَا حَلَّتْ لِي، أَرْضَعَتْنِي وَأَبَاهَا ثُوَيْبَةُ، فَلاَ تَعْرِضْنَ عَلَىَّ بَنَاتِكُنَّ وَلاَ أَخَوَاتِكُنَّ
Law lam takun rabībatī mā ḥallat lī, arḍaʿatnī wa abāhā Thuwaybah. Fa-lā taʿriḍna ʿalayya banātikunna wa lā akhawātikunn
Even if she were not my stepdaughter she would not be lawful for me. Thuwaybah nursed both me and her father. So do not offer me your daughters or your sisters.
Where this comes from
Umm Habibah said she would rather share him with her own sister than a stranger. He answered that the sister was not lawful. Then she mentioned another name she had heard of, and he explained that one too. Milk creates the same barrier as blood, and one woman decades earlier had made two families into one.
The full narration
Narrated Um Habiba:I said, "O Allah's Messenger (ﷺ)! Do you like to have (my sister) the daughter of Abu Sufyan?" The Prophet (ﷺ) said, "What shall I do (with her)?" I said, "Marry her." He said, "Do you like that?" I said, "(Yes), for even now I am not your only wife, so I like that my sister should share you with me." He said, "She is not lawful for me (to marry)." I said, "We have heard that you want to marry." He said, "The daughter of Um Salama?" I said, "Yes." He said, "Even if she were not my stepdaughter, she should be unlawful for me to marry, for Thuwaiba suckled me and her father (Abu Salama). So you should neither present your daughters, nor your sisters, to me
Source
Ṣaḥīḥ al-Bukhārī · 5106
- Agreed upon as authentic (al-Bukhārī / Muslim)