(39) باب لاَ تُؤْخَذُ فِي الصَّدَقَةِ هَرِمَةٌ وَلاَ ذَاتُ عَوَارٍ وَلاَ تَيْسٌ إِلاَّ مَا شَاءَ الْمُصَدِّقُ
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).
Reference: Sahih al-Bukhari 1455
In-book reference: Book 24, Hadith 59
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 24, Hadith 535