Abu Sa'id al-Khudri reported Allah's Messenger (way peace be upon him) as saying:
No 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).
Reference: Sahih Muslim 979 a
In-book reference: Book 12, Hadith 1
USC-MSA web (English) reference(deprecated numbering scheme): Book 5, Hadith 2134