Sahih Muslim 1621
Sahih Muslim · Book 24: The Book of Gifts · It Is Disliked For A Man To Buy What He Gave In Charity From The One To Whom He Gave It
Salim reported from Ibn Umar (Allah be pleased with them) that 'Umar donated a horse in the path of Allah and then found it being sold, and he decided to buy that. He asked Allah's Apostle (ﷺ) about it, whereupon Allah's Messenger (ﷺ) said:
حَدَّثَنَا ابْنُ أَبِي عُمَرَ ، وَعَبْدُ بْنُ حُمَيْدٍ ، - وَاللَّفْظُ لِعَبْدٍ - قَالَ أَخْبَرَنَا عَبْدُ الرَّزَّاقِ ، أَخْبَرَنَا مَعْمَرٌ ، عَنِ الزُّهْرِيِّ ، عَنْ سَالِمٍ ، عَنِ ابْنِ عُمَرَ ، أَنَّ عُمَرَ ، حَمَلَ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ ثُمَّ رَآهَا تُبَاعُ فَأَرَادَ أَنْ يَشْتَرِيَهَا فَسَأَلَ النَّبِيَّ صلى الله عليه وسلم فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " لاَ تَعُدْ فِي صَدَقَتِكَ يَا عُمَرُ " .
Umar, do not get back what you gave as charity.