‘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 <i>sadaqa</i> even if he gives it to you for a dirham, for the one who takes back what he gave as <i>sadaqa</i> is like a dog which returns to its vomit.” A version has, “Do not take back what you gave as <i>sadaqa</i>, 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»