Is a bequest made to the Umm Walad (mother of one's child) valid?

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The bequest made to the Umm Walad is valid because she becomes free at the time the bequest obligation becomes effective. This view is supported by a narration from Umar, may Allah be pleased with him, who bequeathed four thousand to the mothers of his children, narrated by Sa'id. This view is also narrated from Imran ibn al-Husayn, and it is the opinion held by Maymun ibn Mahran, Al-Zuhri, Yahya al-Ansari, Malik, Al-Shafi'i, and Ishaq.