The Book of Gifts
Ṣaḥīḥ Muslim 4163
لاَ تَبْتَعْهُ وَلاَ تَعُدْ فِي صَدَقَتِكَ فَإِنَّ الْعَائِدَ فِي صَدَقَتِهِ كَالْكَلْبِ يَعُودُ فِي قَيْئِهِ
lā tabtaʿhu walā taʿud fī ṣadaqatika fa-inna al-ʿāʾida fī ṣadaqatihi ka-l-kalbi yaʿūdu fī qayʾihi
Transliteration generated from the Arabic vowel marks
The full narration
Umar b. Khattab (Allah be pleased with him) reported:I donated a pedigree horse in the path of Allah. Its possessor made it languish. I thought that he would sell it at a cheap price. I asked Allah's Messenger (ﷺ) about it, whereupon he said: Don't buy it and do not get back your charity, for one who gets back the charity is like a dog who swallows its vomit
Source
Ṣaḥīḥ Muslim · 4163
Ṣaḥīḥ