From Sahih Bukhari (Hadith 7330):
Narrated As-Sa'ib bin Yazid:The Sa' (a kind of measure) during the lifetime of the Prophet (ﷺ) used to be equal to the one Mudd (another kind of measure) and one third of a Mudd which we use today, but the Sa' of today has become large
Reference: Bukhari 7330