Obligatory Charity Tax (Zakat)

Bukhari :: Book 2 :: Volume 24 :: Hadith 566

Narrated 'Abdullah bin 'Umar:

Umar bin Al-Khattab gave a horse in charity in Allah's Cause and later he saw it being sold in the market and intended to purchase it. Then he went to the Prophet and asked his permission. The Prophet said, "Do not take back what you have given in charity." For this reason, Ibn 'Umar never purchased the things which he had given in charity, and in case he had purchased something (unknowingly) he would give it in charity again.