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."
Jami` at-Tirmidhi Book 7, Hadith 52
← Previous Hadith "I was sitting with the Prophet when a woman came to him and said: 'O Messenger of Allah! I gave a slave girl to my mother in charity and she died.' He said: 'Your reward is already established, and your right to inherit her has returned it (that Sadaqah) to you.' She said: 'O Messenger of Allah! There was a month of fasting due on her, shall I perform the fast for her?' He said: 'Fast on her behalf.' She said: 'O Messenger of Allah! She never performed Hajj, shall I perform Hajj for her?' He said: 'Yes, perform Hajj on her behalf.'"

Jami` at-Tirmidhi Book 7, Hadith 51 Next Hadith → a man said: "O Messenger of Allah! My mother died, will it benefit her if I give charity on her behalf?" He said: 'Yes.' He said: 'I have a Makhraf, so bear witness that I have given it in charity on her behalf.'"

Jami` at-Tirmidhi Book 7, Hadith 53