Ibn Umar narrated that: Umar gave a horse to be used in the cause of Allah. Then he saw it being sold, so he wanted to buy it, but the Prophet said: "Do not take back what you have given in charity."

'Abdullah bin Abbas (may Allah be pleased with them) says that the Messenger of Allah (peace and blessings of Allah be upon him) said: "An evil example is not suitable for us; the one who takes back a gift is like a dog that vomits and then licks it." 1.

Narrated from Abdullah bin Umar (may Allah be pleased with them) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The example of a person who gives a gift and then takes it back is like a dog that keeps eating until it is full, then vomits, and then licks it up again." Imam Tirmidhi says:
There are also ahadith on this subject from Ibn Abbas and Abdullah bin Amr (may Allah be pleased with them all).