Allah's Messenger, he is the person who seeks permission to enter your house, whereupon Allah's Messenger (ﷺ) said: I think he is so and so (uncle of Hafsa by reason of fosterage). 'A'isha said: Messenger of Allah, if so and so (her uncle by reason of fosterage) were alive, could he enter my house? Allah's Messenger (ﷺ) said: Yes. Fosterage makes unlawful what consanguinity makes unlawful.
The Book of Suckling - Sahih Muslim 1444a
This narration from Sahih Muslim establishes the fundamental Islamic legal principle regarding foster relationships. The Prophet Muhammad (ﷺ) clarifies that fosterage through suckling creates the same permanent marriage prohibitions as blood relations.
Scholarly Commentary
The hadith demonstrates that when a child is suckled by a woman other than its biological mother (before age two), that woman becomes the child's foster mother. Her children become foster siblings, and her siblings become foster uncles/aunts to the suckled child.
The Prophet's response to 'A'isha confirms that foster uncles would be prohibited from marrying her, just as blood uncles are. This ruling extends to all foster relationships - what is unlawful through blood becomes unlawful through milk.
Classical scholars like Imam Nawawi explained that the minimum number of feedings that establish this relationship is five distinct sucklings, based on consensus of the companions and early scholars. This ruling protects family honor and maintains proper boundaries within Islamic society.
Legal Implications
Foster siblings cannot marry each other, just as biological siblings cannot. Foster parents and children have the same marriage prohibitions as biological parents and children. The mahram relationships created through fosterage are permanent and carry the same legal weight as blood relationships in matters of inheritance, guardianship, and privacy boundaries.