I once gave a horse to be used in the cause of Allah. The man I entrusted it to neglected it, so I wanted to buy it back from him, thinking he would sell it cheaply. I asked the Prophet ﷺ about this, and he said, "Do not buy it, even if he offers it to you for a single dirham, for the one who takes back his charity is like a dog that returns to its vomit."
حملت على فرس في سبيل الله، فأضاعه الذي كان عنده، فأردت أن أشتريه منه، وظننت أنه بائعه برخص، فسألت عن ذلك النبي ﷺ فقال: لا تشتره، وإن أعطاكه بدرهم واحد، فإن العائد في صدقته كالكلب يعود في قيئه.
Malik narrated it in zakat (50) from Zayd bin Aslam, from his father, "I heard Umar bin Khattab," and he mentioned it. And through his route, Bukhari narrated it in gifts (2623), and Muslim in gifts (1620).