Narrated Ibn `Umar: Once `Umar gave a horse in charity to be used in holy fighting. It had been given to him by Allah's Apostle . `Umar gave it to another man to ride. Then `Umar was informed that the man put the horse for sale, so he asked Allah's Messenger (ﷺ) whether he could buy it. Allah's Messenger (ﷺ) replied, "You should not buy it, for you should not take back what you have given in charity."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، حَدَّثَنَا عُبَيْدُ اللَّهِ، قَالَ حَدَّثَنِي نَافِعٌ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـ أَنَّ عُمَرَ، حَمَلَ عَلَى فَرَسٍ لَهُ فِي سَبِيلِ اللَّهِ أَعْطَاهَا رَسُولَ اللَّهِ صلى الله عليه وسلم لِيَحْمِلَ عَلَيْهَا رَجُلاً، فَأُخْبِرَ عُمَرُ أَنَّهُ قَدْ وَقَفَهَا يَبِيعُهَا، فَسَأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم أَنْ يَبْتَاعَهَا فَقَالَ " لاَ تَبْتَعْهَا، وَلاَ تَرْجِعَنَّ فِي صَدَقَتِكَ ".
sunnah.com