Sahih al-Bukhari 1490
Narrated `Umar:
Once I gave a horse in Allah's Cause (in charity) but that person did not take care of it. I intended to 
buy it, as I thought he would sell it at a low price. So, I asked the Prophet (p.b.u.h) about it. He said, 
"Neither buy, nor take back your alms which you have given, even if the seller were willing to sell it 
for one Dirham, for he who takes back his alms is like the one who swallows his own vomit."
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، أَخْبَرَنَا مَالِكُ بْنُ أَنَسٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ أَبِيهِ، قَالَ سَمِعْتُ عُمَرَ ـ رضى الله عنه ـ يَقُولُ حَمَلْتُ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ، فَأَضَاعَهُ الَّذِي كَانَ عِنْدَهُ، فَأَرَدْتُ أَنْ أَشْتَرِيَهُ، وَظَنَنْتُ أَنَّهُ يَبِيعُهُ بِرُخْصٍ، فَسَأَلْتُ النَّبِيَّ صلى الله عليه وسلم فَقَالَ ‏
"‏ لاَ تَشْتَرِ وَلاَ تَعُدْ فِي صَدَقَتِكَ، وَإِنْ أَعْطَاكَهُ بِدِرْهَمٍ، فَإِنَّ الْعَائِدَ فِي صَدَقَتِهِ كَالْعَائِدِ فِي قَيْئِهِ ‏"‏‏.‏