Sahih al-Bukhari 2623
Narrated `Umar bin Al-Khattab:
I gave a horse in Allah's Cause. The person to whom it was given, did not look after it. I intended to 
buy it from him, thinking that he would sell it cheap. When I asked the Prophet (ﷺ) he said, "Don't buy it, 
even if he gives it to you for one Dirham, as the person who takes back what he has given in charity, is 
like a dog that swallows back its vomit."
حَدَّثَنَا يَحْيَى بْنُ قَزَعَةَ، حَدَّثَنَا مَالِكٌ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ أَبِيهِ، سَمِعْتُ عُمَرَ بْنَ الْخَطَّابِ ـ رضى الله عنه ـ يَقُولُ حَمَلْتُ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ، فَأَضَاعَهُ الَّذِي كَانَ عِنْدَهُ، فَأَرَدْتُ أَنْ أَشْتَرِيَهُ مِنْهُ، وَظَنَنْتُ أَنَّهُ بَائِعُهُ بِرُخْصٍ، فَسَأَلْتُ عَنْ ذَلِكَ النَّبِيَّ صلى الله عليه وسلم فَقَالَ ‏
"‏ لاَ تَشْتَرِهِ، وَإِنْ أَعْطَاكَهُ بِدِرْهَمٍ وَاحِدٍ، فَإِنَّ الْعَائِدَ فِي صَدَقَتِهِ كَالْكَلْبِ يَعُودُ فِي قَيْئِهِ ‏"‏‏.‏