Sahih al-Bukhari 2636
Sahih al-Bukhari · Book 51: Gifts · If somebody gives another person a horse (as a gift)
Narrated `Umar bin Al-Khattab:
حَدَّثَنَا الْحُمَيْدِيُّ ، أَخْبَرَنَا سُفْيَانُ ، قَالَ سَمِعْتُ مَالِكًا ، يَسْأَلُ زَيْدَ بْنَ أَسْلَمَ قَالَ سَمِعْتُ أَبِي يَقُولُ، قَالَ عُمَرُ ـ رضى الله عنه ـ حَمَلْتُ عَلَى فَرَسٍ فِي سَبِيلِ اللَّهِ، فَرَأَيْتُهُ يُبَاعُ، فَسَأَلْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم فَقَالَ " لاَ تَشْتَرِ، وَلاَ تَعُدْ فِي صَدَقَتِكَ " .
Once I gave a horse (for riding) in Allah's Cause. Later I saw it being sold. I asked Allah's Messenger (ﷺ) (whether I could buy it). He said, "Don't buy it, for you should not get back what you have given in charity."