Sunan an-Nasai Book 23, Hadith 2513

أَخْبَرَنَا هَنَّادُ بْنُ السَّرِيِّ، عَنْ وَكِيعٍ، عَنْ دَاوُدَ بْنِ قَيْسٍ، عَنْ عِيَاضِ بْنِ عَبْدِ اللَّهِ، عَنْ أَبِي سَعِيدٍ، قَالَ كُنَّا نُخْرِجُ صَدَقَةَ الْفِطْرِ إِذْ كَانَ فِينَا رَسُولُ اللَّهِ صلى الله عليه وسلم صَاعًا مِنْ طَعَامٍ أَوْ صَاعًا مِنْ تَمْرٍ أَوْ صَاعًا مِنْ شَعِيرٍ أَوْ صَاعًا مِنْ أَقِطٍ فَلَمْ نَزَلْ كَذَلِكَ حَتَّى قَدِمَ مُعَاوِيَةُ مِنَ الشَّامِ وَكَانَ فِيمَا عَلَّمَ النَّاسَ أَنَّهُ قَالَ مَا أَرَى مُدَّيْنِ مِنْ سَمْرَاءِ الشَّامِ إِلاَّ تَعْدِلُ صَاعًا مِنْ هَذَا ‏.‏ قَالَ فَأَخَذَ النَّاسُ بِذَلِكَ ‏.‏

It was narrated that Abu Sa'eed said:"We used to pay Salaqatul Fitr when the Messenger of Allah was among us; a Sa' of food, or a Sa' of dates, or a Sa' of barley, or a Sa' of cottage cheese. We continued to do so until Mu'awiyah came from Ash-Sham and one of the things that he taught the people was when he said: I think that two Mudds of wheat from Ash-Sham are equivalent to a Sa' of this, So the people took to that