Sahih al-Bukhari 3002
Sahih al-Bukhari · Book 56: Fighting for the Cause of Allah (Jihaad) · If someone gives his horse for Allah's Cause and then he sees it being sold
Narrated `Abdullah bin `Umar:
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 " .
`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."