Musnad Ahmad 3019

It was narrated from Ibn ‘Abbas (رضي الله عنه) that The daughter of Hamzah was suggested (as a potential wife) to the Prophet (ﷺ) and he said: “She is the daughter of my brother through breastfeeding and she is not permissible for me. What becomes mahram (forbidden for marriage) through breastfeeding is that which becomes mahram through blood ties.”