From Sahih Muslim (Hadith 2281):
Abdullah b. Umar reported that the Messenger of Allah (ﷺ) 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
Reference: Muslim 2281