Hadith: The one who takes back the gift he has given to someone is like one who eats his vomit

عن عبد الله بن عباس رضي الله عنهما مرفوعاً: "العائد في هِبَتِهِ، كالعائد في قَيْئِهِ". وفي لفظ: "فإن الذى يعود في صدقته: كالكلب يَقِئ ُثم يعود في قيئه".
[صحيح] - [متفق عليه]

‘Abdullāh ibn ‘Abbās (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "The one who takes back the gift he has given to someone is like one who eats his vomit." In another wording: "The one who takes back the charity that he gave is like a dog that vomits then eats its vomit."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) drew an analogy to show how repulsive and disgusting it is that a person takes back the gift he has already given to someone. The one who does so is like a dog that vomits then it comes back to its vomit and eats it. This is an indication of the hideousness of such behavior and the meanness of the person who does it.