Mishkat al-Masabih 1954
‘Umar b. al-Rhattab said:
I provided a man with a horse to ride in God’s path, but as the one who had it did not look after it well, I wanted to buy it and thought he would sell it at a cheap price. I therefore asked the Prophet, but he said, 
“Do not buy it, and do not take back what you gave as sadaqa even if he gives it to you for a dirham, for the one who takes back what he gave as sadaqa is like a dog which returns to its vomit.” 
A version has, “Do not take back what you gave as sadaqa , for the one who does so is like one who takes back what he has vomited.” 

(Bukhari and Muslim.)
عَنْ عُمَرَ بْنِ الْخَطَّابِ رَضِيَ اللَّهُ عَنْهُ قَالَ: حَمَلْتُ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ فَأَضَاعَهُ الَّذِي كَانَ عِنْدَهُ فَأَرَدْتُ أَنْ أَشْتَرِيَهُ وَظَنَنْتُ أَنَّهُ يَبِيعُهُ بِرُخْصٍ فَسَأَلْتُ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَقَالَ: «لَا تَشْتَرِهِ وَلَا تَعُدْ فِي صَدَقَتِكَ وَإِنْ أَعْطَاكَهُ بِدِرْهَمٍ فَإِنَّ الْعَائِدَ فِي صَدَقَتِهِ كَالْكَلْبِ يَعُودُ فِي قَيْئِهِ» . وَفِي رِوَايَةٍ: «لَا تَعُدْ فِي صَدَقَتِكَ فَإِنَّ الْعَائِدَ فِي صَدَقَتِهِ كالعائد فِي قيئه»