Language:
كتاب الجهاد والسير
56  
Fighting for the Cause of Allah (Jihaad)
(137)
Chapter: If someone gives his horse for Allah's Cause and then he sees it being sold
(137)
باب إِذَا حَمَلَ عَلَى فَرَسٍ فَرَآهَا تُبَاعُ
Sahih al-Bukhari 3002

Narrated `Abdullah bin `Umar:

`Umar bin Al-Khattab gave a horse to be ridden in Allah's Cause and then he found it being sold. He intended to purchase it. So, he consulted Allah's Messenger (ﷺ) who said, "Don't buy it and don't take back your gift of charity."

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، أَخْبَرَنَا 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 ‏"‏‏.‏
Reference : Sahih al-Bukhari 3002
In-book reference : Book 56, Hadith 211
USC-MSA web (English) reference : Vol. 4, Book 52, Hadith 246
  (deprecated numbering scheme)