Sunan Abi Dawud, Hadith 1589

Narrated ‘Abdallah bin Umar : ‘Umar bin Al Khattab gave a horse as alms in the way of Allah. He then found it being sold, and intended to buy it. So he asked the Messenger of Allah about this. He said Do not buy it, and do not take back your sadaqah.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ، عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، - رضى الله عنه - حَمَلَ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ فَوَجَدَهُ يُبَاعُ فَأَرَادَ أَنْ يَبْتَاعَهُ فَسَأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم عَنْ ذَلِكَ فَقَالَ ‏ "‏ لاَ تَبْتَعْهُ وَلاَ تَعُدْ فِي صَدَقَتِكَ ‏" ‏ ‏.‏

sunnah.com

also referenced under: Hadith 1593 (DarusSalam)