Sahih Al Bukhari (617)

عن عمر بن الخطاب (رضي الله عنه) يقول "حملت على فرس في سبيل الله فأضاعه الذي كان عنده فأردت أن أشتريه منه وظننت أنه بائعه برخص فسألت عن ذلك النبي (صلى الله عليه وسلم) فقال لا تشتره وإن أعطاكه بدرهم واحد فإن العائد في صدقته كالكلب يعود في قيئه"

`Umar ibn Al Khattab narrated: I gave a horse in Allah's Cause. The person to whom it was given, did not look after it. I intended to buy it from him, thinking that he would sell it cheap. When I asked the Prophet (peace be upon him) he said: "Don't buy it, even if he gives it to you for one Dirham, as the person who takes back what he has given in charity, is like a dog that swallows back its vomit."