(4) باب زَكَاةِ الْفِطْرِ عَلَى الْمُسْلِمِينَ مِنَ التَّمْرِ وَالشَّعِيرِ
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: Sahih Muslim 984 d
In-book reference: Book 12, Hadith 19
USC-MSA web (English) reference(deprecated numbering scheme): Book 5, Hadith 2152