حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ بْنِ قَعْنَبٍ، حَدَّثَنَا مَالِكُ بْنُ أَنَسٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ أَبِيهِ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، قَالَ حَمَلْتُ عَلَى فَرَسٍ عَتِيقٍ فِي سَبِيلِ اللَّهِ فَأَضَاعَهُ صَاحِبُهُ فَظَنَنْتُ أَنَّهُ بَائِعُهُ بِرُخْصٍ فَسَأَلْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم عَنْ ذَلِكَ فَقَالَ ‏"‏ لاَ تَبْتَعْهُ وَلاَ تَعُدْ فِي صَدَقَتِكَ فَإِنَّ الْعَائِدَ فِي صَدَقَتِهِ كَالْكَلْبِ يَعُودُ فِي قَيْئِهِ ‏"‏ ‏.‏

Umar b. Khattab (Allah be pleased with him) reported: I donated a pedigree horse in the path of Allah. Its possessor made it languish. I thought that he would sell it at a cheap price. I asked Allah's Messenger (PBUH) about it, whereupon he said: Don't buy it and do not get back your charity, for one who gets back the charity is like a dog who swallows its vomit. Translation Not Available

Book reference: Sahih Muslim Book 24 Hadith 3949
Web reference: Sahih Muslim Book 12 Hadith 1620 a