Obligatory Charity Tax (Zakat)
Bukhari :: Book 2 :: Volume 24 :: Hadith 535
Narrated Anas:
Abu Bakr wrote to me what Allah had ordered His Apostle (about Zakat) which goes: Neither an old nor a defected animal, nor a male-goat may be taken as Zakat except if the Zakat collector wishes (to take it).