His words, “by lineage or suckling”: lineage is well known. Allah has enumerated the women prohibited through lineage in the Quran, saying:
“Prohibited to you are your mothers, your daughters, your sisters, your paternal aunts, your maternal aunts, your brother’s daughters, and your sister’s daughters.” [al-Nisāʾ 4:23]
They are seven: one’s mother, however far ascending; one’s daughter, however far descending; one’s sister; one’s paternal aunt, however far ascending; one’s maternal aunt, however far ascending; one’s brother’s daughter, however far descending; and one’s sister’s daughter, however far descending. These are the seven.
Their seven counterparts through suckling are likewise forbidden, for the Prophet, may Allah’s peace and blessings be upon him, said: “What is forbidden through suckling is the same as what is forbidden through lineage.” (1) This explains the Quran, for the Quran says:
“And your mothers who nursed you and your sisters through suckling.” [al-Nisāʾ 4:23]
The sunnah then came to clarify this. Thus, one’s mother through suckling, however far ascending; one’s daughter through suckling, however far descending; one’s sister through suckling; one’s paternal aunt through suckling, however far ascending; one’s maternal aunt through suckling, however far ascending; one’s brother’s daughter through suckling, however far descending; and one’s sister’s daughter through suckling, however far descending—all are forbidden.
Thus, if a man says to his wife, “You are to me like the back of my mother through suckling,” he has pronounced ẓihār, because the Messenger, may Allah’s peace and blessings be upon him, said: “What is forbidden through suckling is the same as what is forbidden through lineage.” Although, without doubt, likening her to the back of one’s mother by lineage is more abhorrent than likening her to the back of one’s mother through suckling, and a niece through suckling is not like a niece by lineage, the ruling is nevertheless the same, since the Prophet, may Allah’s peace and blessings be upon him, said: “What is forbidden through suckling is the same as what is forbidden through lineage.”