It was narrated from 'Umar, meaning, from his father, his grandfather 'Umar, that : he gave a horse in charity at the time of the Messenger of Allah (ﷺ), then he saw its owner selling it for a low price. He went to the Prophet (ﷺ) and asked him about that, and he said: “Do not buy what you gave in charity.”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا تَمِيمُ بْنُ الْمُنْتَصِرِ الْوَاسِطِيُّ، حَدَّثَنَا إِسْحَاقُ بْنُ يُوسُفَ، عَنْ شَرِيكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ عُمَرَ بْنِ عَبْدِ اللَّهِ بْنِ عُمَرَ يَعْنِي، عَنْ أَبِيهِ، عَنْ جَدِّهِ، عُمَرَ أَنَّهُ تَصَدَّقَ بِفَرَسٍ عَلَى عَهْدِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَأَبْصَرَ صَاحِبَهَا يَبِيعُهَا بِكَسْرٍ فَأَتَى النَّبِيَّ صلى الله عليه وسلم فَسَأَلَهُ عَنْ ذَلِكَ فَقَالَ " لاَ تَبْتَعْ صَدَقَتَكَ " .
sunnah.com