Sahih al-Bukhari · No. 2636Chapter 51
" 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."