Muwatta Malik #641
Zakat
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، حَمَلَ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ فَأَرَادَ أَنْ يَبْتَاعَهُ فَسَأَلَ عَنْ ذَلِكَ رَسُولَ اللَّهِ صلى الله عليه وسلم فَقَالَ
" لاَ تَبْتَعْهُ وَلاَ تَعُدْ فِي صَدَقَتِكَ " .
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.