Sahih Bukhari | Volume 8 | Hadith Number 703

Narrated by Al-Ju'aid bin Abdur-Rahman
As-Sa'ib bin Yazid said, "The Sa' at the time of the Prophet was equal to one Mudd plus one-third of a Mudd of your time, and then it was increased in the time of Caliph 'Umar bin 'Abdul Aziz."