Sahih al-Bukhari, Book 56, Hadith 180 (In-Book) ⓘ

Narrated `Abdullah bin `Umar: `Umar gave a horse to be used in Allah's Cause, but later on he found it being sold. So, he intended to buy it and asked Allah's Messenger who said, "Don't buy it and don't take back your gift of charity."

حَدَّثَنَا إِسْمَاعِيلُ، قَالَ حَدَّثَنِي مَالِكٌ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ـ رضى الله عنهما أَنَّ عُمَرَ بْنَ الْخَطَّابِ، حَمَلَ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ، فَوَجَدَهُ يُبَاعُ، فَأَرَادَ أَنْ يَبْتَاعَهُ، فَسَأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم فَقَالَ ‏ "‏ لاَ تَبْتَعْهُ، وَلاَ تَعُدْ فِي صَدَقَتِكَ ‏" ‏‏.‏