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).
— Sahih Bukhari, Volume 2, Book 24: Obligatory Charity Tax (Zakat), Hadith #535