Abdullah ibn al-Zubayr · The Book of Marriage

"I heard 'Aishah say: The Messenger of Allah said: 'What becomes unlawful (for marriage) through breast-feeding is that which becomes unlawful through birth.'"

"I said: 'O Messenger of Allah, why do you choose wives from among Quraish and not from among us?' He said: 'Do you have anyone in mind?' I said: 'Yes, the daughter of Hamzah.' The Messenger of Allah said: 'She is not permissible for me (to marry); she is the daughter of my brother through breast-feeding.'"