Hadith: When the Messenger of Allah (may Allah's peace and blessings be upon him) was among us, we used to give out Zakat al-Fitr on behalf of every young and old person, free or slave, as one Sā‘ of food, or one Sā‘ of Aqit (dried yogurt), or one Sā‘ of barley, or one Sā‘ of dates, or one Sā‘ of raisins

عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ رضي الله عنه قَالَ:
كُنَّا نُخْرِجُ إِذْ كَانَ فِينَا رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ زَكَاةَ الْفِطْرِ، عَنْ كُلِّ صَغِيرٍ وَكَبِيرٍ، حُرٍّ أَوْ مَمْلُوكٍ، صَاعًا مِنْ طَعَامٍ، أَوْ صَاعًا مِنْ أَقِطٍ، أَوْ صَاعًا مِنْ شَعِيرٍ، أَوْ صَاعًا مِنْ تَمْرٍ، أَوْ صَاعًا مِنْ زَبِيبٍ، فَلَمْ نَزَلْ نُخْرِجُهُ حَتَّى قَدِمَ عَلَيْنَا مُعَاوِيَةُ بْنُ أَبِي سُفْيَانَ رضي الله عنه حَاجًّا، أَوْ مُعْتَمِرًا فَكَلَّمَ النَّاسَ عَلَى الْمِنْبَرِ، فَكَانَ فِيمَا كَلَّمَ بِهِ النَّاسَ أَنْ قَالَ: إِنِّي أَرَى أَنَّ مُدَّيْنِ مِنْ سَمْرَاءِ الشَّامِ، تَعْدِلُ صَاعًا مِنْ تَمْرٍ، فَأَخَذَ النَّاسُ بِذَلِكَ، قَالَ أَبُو سَعِيدٍ: فَأَمَّا أَنَا فَلَا أَزَالُ أُخْرِجُهُ كَمَا كُنْتُ أُخْرِجُهُ، أَبَدًا مَا عِشْتُ.
[صحيح] - [متفق عليه] - [صحيح مسلم: 985]

Abu Sa‘īd al-Khudri (may Allah be pleased with him) reported:
When the Messenger of Allah (may Allah's peace and blessings be upon him) was among us, we used to give out Zakat al-Fitr on behalf of every young and old person, free or slave, as one Sā‘ of food, or one Sā‘ of Aqit (dried yogurt), or one Sā‘ of barley, or one Sā‘ of dates, or one Sā‘ of raisins. We continued giving it out until Mu‘āwiyah ibn Abi Sufyān (may Allah be pleased with him) came to us for Hajj or ‘Umrah. He addressed the people from the pulpit, and among the things he said to the people was: Verily, I see that two Mudds of the Syrian wheat are equivalent to one Sā‘ of dates. So, people started applying this. Abu Sa‘īd said: As for me, I will continue to give it as I used to give it for as long as I live.
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Muslim - 985]

Explanation

Muslims used to give out Zakat al-Fitr on behalf of every young and old person as one Sā‘ of food during the time of the Prophet (may Allah's peace and blessings be upon him) and during the time of the Rightly-Guided Caliphs. Their food was barley, (raisins): dry grapes, (Aqit): dried yogurt, and dates. One Sā‘ is equivalent to four Mudds, and one Mudd is equal to two handfuls of an average person. When Mu‘āwiyah (may Allah be pleased with him) came to Madīnah as a caliph and the Syrian wheat became abundant, he delivered a speech and said: I see that two Mudds of the Syrian wheat (half a Sā‘) is equivalent to one Sā‘ of dates, so the people adopted this opinion. Abu Sa‘īd al-Khudri (may Allah be pleased with him) said: As for me, I will continue to give it as I used to give it during the Prophet's lifetime for as long as I live.