Sunan An Nasai 2616

أَخْبَرَنَا مُحَمَّدُ بْنُ سَلَمَةَ، ‏‏‏‏‏‏وَالْحَارِثُ بْنُ مِسْكِينٍ قراءة عليه وأنا أسمع، ‏‏‏‏‏‏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 صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَقَالَ:‏‏‏‏ لَا تَشْتَرِهِ وَإِنْ أَعْطَاكَهُ بِدِرْهَمٍ، ‏‏‏‏‏‏فَإِنَّ الْعَائِدَ فِي صَدَقَتِهِ كَالْكَلْبِ يَعُودُ فِي قَيْئِهِ .

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. '