Sahih al-Bukhari 2971
Sahih al-Bukhari · Book 56: Fighting for the Cause of Allah (Jihaad) · The wages given to fight on somebody else's behalf
Narrated `Abdullah bin `Umar:
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 " .
`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 (ﷺ) who said, "Don't buy it and don't take back your gift of charity."