Musnad Ahmad 1156:160

Ibn Numayr narrated to us, from Hisham, from his father, from Zaynab bint Umm Salamah, from Umm Salamah, that Umm Habibah said: "O Messenger of Allah, do you have any interest in my sister, the daughter of Abu Sufyan?" He said: "And what would I do?" She said: "You marry her." He said: "And is that more beloved to you?" She said: "Yes, I am not the only one for you, and the most beloved of those who share the good with me is my sister." He said: "She is not lawful for me." I said: "It has reached me that you are proposing to Durrah bint Abi Salamah." He said: "The daughter of Umm Salamah?" She said: "Yes." He said: "By Allah, even if she were not my stepdaughter under my guardianship, she would not be lawful for me. She is the daughter of my brother through breastfeeding; Thuwaybah nursed me and her father. So do not present your daughters to me, nor your sisters."