Sahih al-Bukhari 6713
Narrated Nafi`:
Ibn `Umar used to give the Zakat of Ramadan (Zakat-al-Fitr) according to the Mudd of the Prophet, 
the first Mudd, and he also used to give things for expiation for oaths according to the Mudd of the 
Prophet. Abu Qutaiba said, "Malik said to us, 'Our Mudd (i.e., of Medina) is better than yours and we 
do not see any superiority except in the Mudd of the Prophet!' Malik further said, to me, 'If a ruler 
came to you and fixed a Mudd smaller than the one of the Prophet, by what Mudd would you measure 
what you give (for expiation or Zakat-al-Fitr?' I replied, 'We would give it according to the Mudd of 
the Prophet' On that, Malik said, 'Then, don't you see that we have to revert to the Mudd of the 
Prophet ultimately?'"
حَدَّثَنَا مُنْذِرُ بْنُ الْوَلِيدِ الْجَارُودِيُّ، حَدَّثَنَا أَبُو قُتَيْبَةَ ـ وَهْوَ سَلْمٌ ـ حَدَّثَنَا مَالِكٌ، عَنْ نَافِعٍ، قَالَ كَانَ ابْنُ عُمَرَ يُعْطِي زَكَاةَ رَمَضَانَ بِمُدِّ النَّبِيِّ صلى الله عليه وسلم الْمُدِّ الأَوَّلِ، وَفِي كَفَّارَةِ الْيَمِينِ بِمُدِّ النَّبِيِّ صلى الله عليه وسلم‏.‏ قَالَ أَبُو قُتَيْبَةَ قَالَ لَنَا مَالِكٌ مُدُّنَا أَعْظَمُ مِنْ مُدِّكُمْ وَلاَ نَرَى الْفَضْلَ إِلاَّ فِي مُدِّ النَّبِيِّ صلى الله عليه وسلم‏.‏ وَقَالَ لِي مَالِكٌ لَوْ جَاءَكُمْ أَمِيرٌ فَضَرَبَ مُدًّا أَصْغَرَ مِنْ مُدِّ النَّبِيِّ صلى الله عليه وسلم بِأَىِّ شَىْءٍ كُنْتُمْ تُعْطُونَ قُلْتُ كُنَّا نُعْطِي بِمُدِّ النَّبِيِّ صلى الله عليه وسلم قَالَ أَفَلاَ تَرَى أَنَّ الأَمْرَ إِنَّمَا يَعُودُ إِلَى مُدِّ النَّبِيِّ صلى الله عليه وسلم‏.‏