Bulugh al-Maram, Book 15, Hadith 1479 (English)

Ibn ’Abbas (RAA) narrated that Allah’s Messenger said: “When a man’s slave-woman bears him a child she becomes free at his death.” Related by Ibn Majah and Al-Hakim with a weak chain of narrators.

وَعَنْ اِبْنِ عَبَّاسٍ قَالَ: قَالَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-{ أَيُّمَا أَمَةٍ وَلَدَتْ مِنْ سَيِّدِهَا, فَهِيَ حُرَّةٌ بَعْدَ مَوْتِهِ } أَخْرَجَهُ اِبْنُ مَاجَهْ, وَالْحَاكِمُ بِإِسْنَادٍ ضَعِيف ٍ 1‏.‏ ‏1 ‏- ضعيف.‏ رواه ابن ماجه (2515)‏، والحاكم (2 / 19)‏.‏

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also referenced under: Book 15, Hadith 1435 (In-Book)