Musannaf Ibn Abi Shayba 9:1128

Ibn ‘Ulayyah narrated to us, from Ayyub, from Nafi’, who said: When ‘Aishah wanted someone to enter upon her, she would order that he be breastfed. She ordered Umm Kulthum to breastfeed Salim ten times. She breastfed him three times, then she became ill, so he would not enter upon her. She ordered Fatimah bint ‘Amr to breastfeed ‘Asim ibn Sa’id, a freed slave of theirs, and she breastfed him ten times, so he would enter upon her.