Sahih Bukhari Volume 008, Book 079, Hadith Number 704.
Narated By 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?'"