مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3120
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated from Ibn 'Abbas (رضي الله عنه) that It was suggested to the Messenger of Allah (ﷺ) that he should marry the daughter of Hamzah and he said: "She is the daughter of my brother through breastfeeding. What becomes mahram (forbidden for marriage) through breastfeeding is that which becomes mahram through blood ties.”
Musnad Ahmad 3144Its isnad is saheeh, al-Bukhari (2645) and Muslim (1447)]