The second class of prohibitions relates to fosterage, but while the Holy Qur’ān mentions only the foster mother and the foster-sister, Ḥadīth is clear on the point that all those relations that are prohibited in marriage on account of consanguinity are prohibited also on account of fosterage16 (Bu. 67:21). Thus the foster-uncle of Ḥafṣah was declared to fall within the prohibitions of the Holy Qur’ān, and a daughter of Ḥamzah who, though an uncle of the Holy Prophet, was also his foster-brother, was regarded as forbidden for the Holy Prophet (Bu. 67:21). Certain foster-relations may, however, be complete strangers, though they would not be so in the case of blood relations. For instance, the brother’s mother is either the mother or stepmother of a man, and in both cases she is among the prohibited relations, but the foster-brother’s mother may be quite a stranger and in that case is not prohibited.
As to what constitutes fosterage (radzā‘ah or giving suck), there is a very slight difference of opinion. A child is recognized as a suckling only up to the age of two years, in the Holy Qur’ān (2:233), and there is no difference on this point. Ḥadīth lays down that foster-relationship is not established unless the child is suckled when hungry (Bu. 67:22). Imām Abū Ḥanīfah, however, considers the child’s being given suck only once as sufficient to establish foster-relationship; Imām Shāfi‘ī is of opinion that he must have been suckled four times, while the Shī‘ahs consider it necessary that he should have been suckled at least for twenty-four hours.
The third class of prohibitions is that which arises from affinity, and here, too, the Jurists have expanded the conception of relationship in the same manner as in the case of consanguinity. Thus wife’s mother includes wife’s mother’s mother, and so on; wife’s daughter includes her daughter’s daughter;17 son’s wife18 includes son’s son’s wife. Step-mothers are expressly prohibited in the Holy Qur’ān: “And marry not women whom your fathers married” (4:22). Jurists also lay down that a man’s unlawful connection with a woman included her in the category of a wife so far as prohibitions arising from the wife’s connection are concerned.
The last prohibition relates to the gathering together of two sisters as co-wives. Ḥadīth expands this conception and prohibits the
16 Sayyid Amīr ‘Alī mentions the following exceptions to this rule as recognized by the Ahl Sunnah: (i) The marriage of the father of the child with the mother of his child’s foster-mother; (ii) with her daughter; (iii) the marriage of the foster-mother with the brother of the child whom she has fostered; (iv) the marriage with foster-mother of an uncle or aunt. And he adds: “According to the Durr al-Mukhtār there are twenty-one exceptions to this rule. For example, the foster-mother of a child and her mother are lawful to the child’s grandfather. Similarly a man may marry his brother’s or sister’s foster-mother; his son’s foster-sister; paternal or maternal uncle’s foster mother; son’s paternal aunt by fosterage, etc. The husband of the foster-mother may marry the natural mother or sister of his wife’s foster-child.” The Shī‘ahs recognize no exception.
17 The wife’s mother is prohibited unconditionally while the wife’s daughter is prohibited only in case a man has gone in to his wife; see 4:23 quoted above.
18 It is expressly laid down in the Holy Qur’ān that the son whose wife is forbidden is one from one’s own loins. The adopted son who is really not a son at all, is thus excluded.