I gave a horse to be used in Allah's Cause, but later on I saw it being sold. I asked the Prophet ﷺ whether I could buy it. He said, "Don't buy it and don't take back your gift of charity."  

عُمَرُ بْنُ الْخَطَّابِ ؓ حَمَلْتُ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ فَرَأَيْتُهُ يُبَاعُ فَسَأَلْتُ النَّبِيَّ ﷺ آشْتَرِيهِ فَقَالَ لاَ تَشْتَرِهِ وَلاَ تَعُدْ فِي صَدَقَتِكَ  

ʿUmar gave a horse to be used in Allah's Cause, but later on he found it being sold. So, he intended to buy it and asked Messenger of Allah ﷺ who said, "Don't buy it and don't take back your gift of charity."  

أَنْ يَبْتَاعَهُ فَسَأَلَ رَسُولَ اللَّهِ ﷺ فَقَالَ لاَ تَبْتَعْهُ وَلاَ تَعُدْ فِي صَدَقَتِكَ  

Messenger of Allah ﷺ said, "Were it not for the fear that it would be difficult for my followers, I would not have remained behind any Sariya, (army-unit) but I don't have riding camels and have no other means of conveyance to carry them on, and it is hard for me that my companions should remain behind me. No doubt I wish I could fight in Allah's Cause and be martyred and come to life again to be martyred and come to life once more."  

قَالَ رَسُولُ اللَّهِ ﷺ لَوْلاَ أَنْ أَشُقَّ عَلَى أُمَّتِي مَا تَخَلَّفْتُ عَنْ سَرِيَّةٍ وَلَكِنْ لاَ أَجِدُ حَمُولَةً وَلاَ أَجِدُ مَا أَحْمِلُهُمْ عَلَيْهِ وَيَشُقُّ عَلَىَّ أَنْ يَتَخَلَّفُوا عَنِّي وَلَوَدِدْتُ أَنِّي قَاتَلْتُ فِي سَبِيلِ اللَّهِ فَقُتِلْتُ ثُمَّ أُحْيِيتُ ثُمَّ قُتِلْتُ ثُمَّ أُحْيِيتُ