Sahih Al Bukhari Hadith Number 1455

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).

24  Obligatory Charity Tax (Zakat)
Hadith number 1455
Volume no 2
Chapter: 39 (Neither an old, nor a defective animal, nor a male-goat may be taken as Zakat)