Maliks Muwatta Book 17, Hadith Number 50.

Yahya related to me from Zayd ibn Aslam that his father said that he had heard Umar ibn al-Khattab say, “I once gave a noble horse to carry somebody in the way of Allah, and the man neglected it. I wished to buy it back from him and I thought that he would sell it cheaply. I asked the Messenger of Allah, may Allah bless him and grant him peace, about it and he said, ‘Do not buy it, even if he gives it to you for one dirham, for someone who takes back his sadaqa is like a dog swallowing its own vomit.'”