It was narrated from Ibn 'Abbas (رضي الله عنه) that

‘Ali spoke to the Prophet (ﷺ) about the daughter of Hamzah and mentioned how beautiful she was. The Messenger of Allah (ﷺ) said: "She is the daughter of my brother through breastfeeding." Then the Prophet of Allah (ﷺ) said: "Do you not know that Allah, may He be glorified and exalted, makes mahram (forbidden for marriage) through breastfeeding that which He makes mahram through blood ties?"