كتاب الهبات
24  
The Book of Gifts
(1)
Chapter: It Is Disliked For A Man To Buy What He Gave In Charity From The One To Whom He Gave It
(1)
باب كَرَاهَةِ شِرَاءِ الإِنْسَانِ مَا تَصَدَّقَ بِهِ مِمَّنْ تَصَدَّقَ عَلَيْهِ ‏‏
Sahih Muslim 1621 c

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:

Umar, do not get back what you gave as charity.
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 ‏"‏ ‏.‏
Reference : Sahih Muslim 1621c
In-book reference : Book 24, Hadith 7
USC-MSA web (English) reference : Book 12, Hadith 3954
  (deprecated numbering scheme)