Sahih al-Bukhari, Book 51, Hadith 68 (In-Book) ⓘ

Narrated `Umar bin Al-Khattab: Once I gave a horse (for riding) in Allah's Cause. Later I saw it being sold. I asked Allah's Messenger (whether I could buy it). He said, "Don't buy it, for you should not get back what you have given in charity."

حَدَّثَنَا الْحُمَيْدِيُّ، أَخْبَرَنَا سُفْيَانُ، قَالَ سَمِعْتُ مَالِكًا، يَسْأَلُ زَيْدَ بْنَ أَسْلَمَ قَالَ سَمِعْتُ أَبِي يَقُولُ، قَالَ عُمَرُ ـ رضى الله عنه ـ حَمَلْتُ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ، فَرَأَيْتُهُ يُبَاعُ، فَسَأَلْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم فَقَالَ ‏ "‏ لاَ تَشْتَرِ، وَلاَ تَعُدْ فِي صَدَقَتِكَ ‏" ‏‏.‏