(37) باب إِذَا حَمَلَ رَجُلٌ عَلَى فَرَسٍ فَهْوَ كَالْعُمْرَى وَالصَّدَقَةِ
Narrated `Umar bin Al-Khattab:
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."
Reference: Sahih al-Bukhari 2636
In-book reference: Book 51, Hadith 68
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 47, Hadith 804