Muwatta Malik #641
Zakat
Yahya related to me from Malik from Nafi from Abdullah ibn Umar that Umar ibn al-Khattab gave a horse to carry some one in the way of Allah, and then he wished to buy it back. So he asked the Messenger of Allah, may Allah bless him and grant him peace, about it, and he said, "Do not buy or take back your sadaqa."
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About Muwatta Malik
This hadith is #641 from Muwatta Malik, in the Book of Zakat. Muwatta Malik contains 1,985 hadiths across 61 books.