Sunan an-Nasa'i, Volume 3, Book 23, Hadith 2617 (DarusSalam) ⓘ

It was narrated from 'Umar that: he gave someone a horse to ride in the cause of Allah, then he saw it offered for sale and wanted to buy it. The Prophet said to him: "Do not think of buying back what you have given in charity."

أَخْبَرَنَا هَارُونُ بْنُ إِسْحَاقَ، قَالَ حَدَّثَنَا عَبْدُ الرَّزَّاقِ، عَنْ مَعْمَرٍ، عَنِ الزُّهْرِيِّ، عَنْ سَالِمِ بْنِ عَبْدِ اللَّهِ، عَنْ أَبِيهِ، عَنْ عُمَرَ، أَنَّهُ حَمَلَ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ فَرَآهَا تُبَاعُ فَأَرَادَ شِرَاءَهَا فَقَالَ لَهُ النَّبِيُّ صلى الله عليه وسلم ‏ "‏ لاَ تَعْرِضْ فِي صَدَقَتِكَ ‏" ‏ ‏.‏