Sahih Muslim Book 005, Hadith Number 2137.

Chapter : No Zakat is due from a Muslim or his slave or his horse.

Abu Sa'id al-Khudri reported Allah's Messenger (may peace be upon him) as saying: Sadaqa (zakat) is payable on less than five wasqs of (dates or grains), on less than five camel-heads, and on less than five uqiyas (of silver).