Lady Aisha (may Allah be pleased with her) reported that her (foster) uncle, the brother of Abu Quais (Aflah), came to her after the command of hijab, seeking permission to enter. Lady Aisha (may Allah be pleased with her) refused to grant permission and said that she would seek permission from the Messenger of Allah (peace and blessings be upon him). Then, when the Messenger of Allah (peace and blessings be upon him) arrived, Lady Aisha (may Allah be pleased with her) mentioned this to him and said: "The brother of my foster uncle Abu Quais came (and sought permission to enter), so I sent him back until I could ask you?" The Prophet (peace and blessings be upon him) said: "Is he not your foster uncle?" She replied: "The woman breastfed me, not the man (so how is the prohibition established for the man, i.e., Abu Quais or his brother)?" The Prophet (peace and blessings be upon him) said: "He (Aflah) is your uncle; he may enter upon you." Urwah said: And Lady Aisha (may Allah be pleased with her) used to say: "Whatever is made unlawful by birth is also made unlawful by breastfeeding."
Hadith Takhrijاس روایت کی سند صحیح ہے اور حدیث متفق علیہ ہے۔ دیکھئے: [بخاري 2644] ، [مسلم 1445] )، [ترمذي 1148] ، [أبويعلی 4501] ، [ابن حبان 4109]
Brief Explanation
(Commentary on Hadith 2284)
From this hadith, it is understood that the husband of the woman whose milk a child drinks becomes the father of the one who drinks the milk. Now, whatever relationships become prohibited (haram) from the side of the mother and father, those same relationships will also become prohibited through milk (i.e., through breastfeeding).
Aflah became the uncle of Sayyidah Aisha radi Allahu anha because Sayyidah Aisha radi Allahu anha had drunk the milk of the wife of Abu Al-Qais.
And in the production of milk, the reproductive fluids (nutfah) of both the man and the woman are involved; therefore, the relationship of breastfeeding (rada‘ah) is established from both sides, and thus the prohibition (of marriage) is also established.
(Sharh Bulugh al-Maram by Shaykh Safiur Rahman Mubarakpuri rahimahullah).