Musnad Ahmad 258
It was narrated from Zaid bin Aslam from his father from `Umar that:
He found a horse that he had given to someone (for jihad) for the sake of Allah being offered for sale in the marketplace, and he wanted to buy it. He asked the Prophet (ﷺ) [about that] and he told him not to do that and said: “Do not take back your charity.`
حَدَّثَنَا وَكِيعٌ، حَدَّثَنَا هِشَامُ بْنُ سَعْدٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ أَبِيهِ، عَنْ عُمَرَ، رَضِيَ اللَّهُ عَنْهُ أَنَّهُ وَجَدَ فَرَسًا كَانَ حَمَلَ عَلَيْهَا فِي سَبِيلِ اللَّهِ تُبَاعُ فِي السُّوقِ فَأَرَادَ أَنْ يَشْتَرِيَهَا فَسَأَلَ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَنَهَاهُ وَقَالَ لَا تَعُودَنَّ فِي صَدَقَتِكَ‏.‏