Sahih al-Bukhari 6713

Malik related that Nafi' said, "Ibn 'Umar used to pay the zakat of Ramadan (zakat al-fitr) using the original mudd of the Prophet, may Allah bless him and grant him peace, and he also used the mudd of the Prophet, may Allah bless him and grant him peace, for expiating the oath. Abu Qutayba said, "Malik said to us, 'Our mudd is larger than your mudd. We do not see any superiority except in the mudd of the Prophet.' Malik said to me, 'If a ruler came to you and set a mudd smaller than the mudd of the Prophet, may Allah bless him and grant him peace, what would you use to pay it?' I answered, 'We would pay by the mudd of the Prophet, may Allah bless him and grant him peace. He said, 'Do you not see that the business reverts back to the mudd of the Prophet, may Allah bless him and grant him peace?'"