Musnad Ahmad 1167:83

'Affan narrated to us, he said: Hammad ibn Salamah narrated to us, he said: Hisham ibn 'Urwah narrated to us, from 'Urwah, from Asma' bint Abi Bakr, she said: My mother came to me while she was desirous (of receiving something), and she was a polytheist during the time of the Quraysh and their treaty which was between them and the Messenger of Allah, may Allah bless him and grant him peace. I said: "O Messenger of Allah, my mother has come to me while she is desirous, and she is a polytheist; should I maintain ties with her?" He said: "Maintain ties with her." He (the narrator) said: And I think she was her wet nurse.