It was narrated from 'Urwah bin Zubair that Zainab bint Abi Salmah told him that Umm Habibah told her that she said to Messenger of Allah:
"Marry my sister 'Azzah." The Messenger of Allah said: 'Would you like that? "She said: "Yes, O Messenger of Allah. I am not the only one living with you and the one who most deserves to share good thingswith me is my sister." The Messenger of Allah said: "But that is not permissible for me." She said: "But we throught that you wanted to marry Durrah bint Abi Salmah." The Messenger of Allah said: The daughter of Umm Salamah?" She said: "Yes" The Messenger of Allah said: "Even if she were not my step-daughter who is under my care, she would not be permissible for me, because she is the daughter of my brother through breastfeeding. Tuwaibah breastfed both her father and I. So do not offer your sisters and daughters to me for marriage."
English reference: Vol. 3, Book 9, Hadith 1939
Arabic reference: Book 9, Hadith 2014