From Sahih Bukhari (Hadith 6712):
Narrated Al-Ju'aid bin `Abdur-Rahman:As-Sa'ib bin Yazid said, "The Sa' at the time of the Prophet (ﷺ) 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
Reference: Bukhari 6712