Sahih Muslim
The Book of Zakat (Kitab Al-Zakat)
Book Number: 005 / Hadith Number: 2152
Abdullah b. Umar reported that the Messenger of Allah (may peace be upon him) ordered the (payment of) Zakat-ul-Fitr one sa' of dates, or one sa' of barley. Ibn 'Umar ('Abdullah b. 'Umar) further said: The people equalised it (then) with two mudds of fine wheat.