the Messenger of Allah (may Allah's peace and blessings be upon him) said about Hamzah's daughter: "It is unlawful for me to marry her. What is unlawful through lineage is also unlawful through breastfeeding, and she is the daughter of my brother from breastfeeding." [Al-Bukhari]

"عن عبد الله بن عباس -رضي الله عنهما- «قال رسول الله -صلى الله عليه وسلم- في بنت حمزة: لا تحل لي، يحرم من الرضاع: ما يحرم من النسب، وهي ابنة أخي من الرضاعة». [صحيح.] - [متفق عليه.]"