It was narrated from Zaid bin Aslam that his father said:

"I heard 'Umar say: 'I gave a horse to someone to ride in the cause of Allah, the Mighty and Sublime, and the one who kept it neglected it. I wanted to buy it back from him, and I thought that he would sell it at a cheap price. I asked the Messenger for Allah about that and he said: Do not buy it, even if he gives it to you for a Dirham. The one who takes back his charity is like the dog that goes back to its own vomit. "'

أَخْبَرَنَا مُحَمَّدُ بْنُ سَلَمَةَ، وَالْحَارِثُ بْنُ مِسْكِينٍ، قِرَاءَةً عَلَيْهِ وَأَنَا أَسْمَعُ، عَنِ ابْنِ الْقَاسِمِ، قَالَ حَدَّثَنَا مَالِكٌ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ أَبِيهِ، قَالَ سَمِعْتُ عُمَرَ، يَقُولُ حَمَلْتُ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ عَزَّ وَجَلَّ فَأَضَاعَهُ الَّذِي كَانَ عِنْدَهُ وَأَرَدْتُ أَنْ أَبْتَاعَهُ مِنْهُ وَظَنَنْتُ أَنَّهُ بَائِعُهُ بِرُخْصٍ فَسَأَلْتُ عَنْ ذَلِكَ رَسُولَ اللَّهِ ﷺ فَقَالَ " لاَ تَشْتَرِهِ وَإِنْ أَعْطَاكَهُ بِدِرْهَمٍ فَإِنَّ الْعَائِدَ فِي صَدَقَتِهِ كَالْكَلْبِ يَعُودُ فِي قَيْئِهِ " .