(4) باب زَكَاةِ الْفِطْرِ عَلَى الْمُسْلِمِينَ مِنَ التَّمْرِ وَالشَّعِيرِ
Ibn 'Umar said that the Messenger of Allah (ﷺ) prescribed the Sadaqa of Ramadan (Sadaqat-al-Fitr) one sa' of dates or one sa' of barley for every free man or a slave, male or female, and then the people equalised (one sa' of dates or barley) with half a sa' of wheat.
Reference: Sahih Muslim 984 c
In-book reference: Book 12, Hadith 18
USC-MSA web (English) reference(deprecated numbering scheme): Book 5, Hadith 2151