Once ʿUmar gave a horse in charity to be used in holy fighting. It had been given to him by Allah's Apostle . ʿUmar gave it to another man to ride. Then ʿUmar was informed that the man put the horse for sale, so he asked Messenger of Allah ﷺ whether he could buy it. Messenger of Allah ﷺ replied, "You should not buy it, for you should not take back what you have given in charity."  

أَنَّ عُمَرَ حَمَلَ عَلَى فَرَسٍ لَهُ فِي سَبِيلِ اللَّهِ أَعْطَاهَا رَسُولَ اللَّهِ ﷺ لِيَحْمِلَ عَلَيْهَا رَجُلاً فَأُخْبِرَ عُمَرُ أَنَّهُ قَدْ وَقَفَهَا يَبِيعُهَا فَسَأَلَ رَسُولَ اللَّهِ ﷺ أَنْ يَبْتَاعَهَا فَقَالَ لاَ تَبْتَعْهَا وَلاَ تَرْجِعَنَّ فِي صَدَقَتِكَ  

Yahya related to me from Malik from Nafi from Abdullah ibn Umar that Umar ibn al-Khattab gave a horse to carry some one in the way of Allah, and then he wished to buy it back. So he asked the Messenger of Allah ﷺ about it, and he said, "Do not buy or take back your sadaqa."  

أَنَّ عُمَرَ بْنَ الْخَطَّابِ حَمَلَ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ فَأَرَادَ أَنْ يَبْتَاعَهُ فَسَأَلَ عَنْ ذَلِكَ رَسُولَ اللَّهِ ﷺ فَقَالَ لاَ تَبْتَعْهُ وَلاَ تَعُدْ فِي صَدَقَتِكَ  

[Machine] That Umar rode on a horse in the path of Allah, and the Messenger of Allah ﷺ gave him a man, so Umar came to the Messenger of Allah ﷺ and said, "Shall I sell the horse on which I rode?" He said, "Do not sell it and do not take back your charity."  

أَنَّ عُمَرَ حَمَلَ عَلَى فَرَسٍ فِي سَبِيلِ اللهِ فَأَعْطَاهُ رَسُولُ اللهِ ﷺ رَجُلًا فَجَاءَ عُمَرُ إِلَى رَسُولِ اللهِ ﷺ فَقَالَ أَبْتَاعُ الْفَرَسَ الَّذِي حَمَلْتُ عَلَيْهِ فَقَالَ لَا تَبْتَعْهُ وَلَا تَرْجِعْ فِي صَدَقَتِكَ