Sahih al-Bukhari, Hadith 1512 ⓘ
Narrated Ibn `Umar: Allah's Messenger has made Sadaqat-ul-Fitr obligatory, (and it was), either one Sa' of barley or one Sa' of dates (and its payment was obligatory) on young and old people, and on free men as well as on slaves.
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، عَنْ عُبَيْدِ اللَّهِ، قَالَ حَدَّثَنِي نَافِعٌ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنه ـ قَالَ فَرَضَ رَسُولُ اللَّهِ صلى الله عليه وسلم صَدَقَةَ الْفِطْرِ صَاعًا مِنْ شَعِيرٍ، أَوْ صَاعًا مِنْ تَمْرٍ عَلَى الصَّغِيرِ وَالْكَبِيرِ وَالْحُرِّ وَالْمَمْلُوكِ.
also referenced under: Book 24, Hadith 113 (In-Book) & Volume 2, Book 25, Hadith 588 (USC-MSA)