Muwatta Malik, Book 38, Hadith 1467 (In-Book)

Malik related to me from Nafi from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace,said, "If a man frees his share of a slave and has enough money to cover the full price of the slave justly evaluated for him, he must buy out his partners so that the slave is completely freed. If he doesn't have the money, he partially frees him.

حَدَّثَنِي مَالِكٌ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ مَنْ أَعْتَقَ شِرْكًا لَهُ فِي عَبْدٍ فَكَانَ لَهُ مَالٌ يَبْلُغُ ثَمَنَ الْعَبْدِ قُوِّمَ عَلَيْهِ قِيمَةَ الْعَدْلِ فَأَعْطَى شُرَكَاءَهُ حِصَصَهُمْ وَعَتَقَ عَلَيْهِ الْعَبْدُ وَإِلاَّ فَقَدْ عَتَقَ مِنْهُ مَا عَتَقَ ‏" ‏ ‏.‏

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also referenced under: Book 38, Hadith 1 (USC-MSA)