Hadith 512810
Chapter 30: Suckling - كتاب الرضاع
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ صَفِيَّةَ بِنْتَ أَبِي عُبَيْدٍ، أَخْبَرَتْهُ أَنَّ حَفْصَةَ أُمَّ الْمُؤْمِنِينَ أَرْسَلَتْ بِعَاصِمِ بْنِ عَبْدِ اللَّهِ بْنِ سَعْدٍ إِلَى أُخْتِهَا فَاطِمَةَ بِنْتِ عُمَرَ بْنِ الْخَطَّابِ تُرْضِعُهُ عَشْرَ رَضَعَاتٍ لِيَدْخُلَ عَلَيْهَا وَهُوَ صَغِيرٌ يَرْضَعُ فَفَعَلَتْ فَكَانَ يَدْخُلُ عَلَيْهَا .
Yahya related to me from Malik from Nafi that Safiyya bint Abi Ubayd told him that Hafsa, umm al-muminin, sent Asim ibn Abdullah ibn Sad to her sister Fatima bint Umar ibn al-Khattab for her to suckle him ten times so that he could come in to see her. She did it, so he used to come in to see her.
Reference Muwatta Malik 512810
USC-MSA web (English) reference : Book 30, Hadith 8