The Book of Suckling - Sahih Muslim 1447a

It was proposed that he (the Holy Prophet) be married to the daughter of Hamza, whereupon he said: She is not lawful for me for she is the daughter of my foster-brother, and that is unlawful by reason of fosterage what is unlawful by reason of genealogy.

Commentary on the Hadith

This noble hadith establishes a fundamental principle in Islamic family law: that foster relationships (radāʿah) create the same permanent marriage prohibitions as blood relationships. The Prophet Muhammad ﷺ clarified that just as one cannot marry one's biological brother's daughter, one cannot marry one's foster brother's daughter.

Hamza ibn Abdul-Muttalib was the Prophet's paternal uncle, but they also shared a foster relationship through the same wet-nurse, Thuwaybah. This dual relationship made Hamza's daughter doubly prohibited - both as the daughter of his paternal uncle and as the daughter of his foster brother.

The ruling demonstrates that fosterage creates permanent mahram relationships, meaning those who become relatives through suckling cannot marry each other, just like blood relatives. This protection of family honor and prevention of confusion in lineage reflects the wisdom of Islamic legislation.

Legal Implications

Fosterage establishes permanent marriage prohibitions with the foster mother, her daughters, sisters, and other relatives through the same lines as blood relations.

The children of foster siblings are considered like the children of biological siblings in terms of marriage prohibition.

This ruling preserves family integrity and prevents the intermingling of lineages that have been united through the sacred bond of fosterage.