Musnad Ahmad 2618

It was narrated from Ibn ‘Abbas (رضي الله عنه) that the daughter of Hamzah was suggested to Prophet (ﷺ) (for marriage) and he said: “She is the daughter of my brother through breastfeeding, and what becomes mahram (forbidden for marriage) through breastfeeding is the same as that which becomes mahram through blood ties." 'Affan said: "And she is not permissible for me.”