During the time of the Messenger of Allah ﷺ, the saa was equivalent to a mudd and a third by your present-day mudd, and it has since been increased.Abu Abdul-Rahman said, Ziyad bin Ayub also narrated this to me.
أخبرنا عمرو بن زرارة قال أنبأنا القاسم وهو ابن مالك عن الجعيد سمعت
كان الصاع على عهد رسول الله ﷺ مدا وثلثا بمدكم اليوم وقد زيد فيه قال أبو عبد الرحمن و حدثنيه زياد بن أيوب
[Sahih] Bukhari narrated it in "Holding Fast to the Book and the Sunnah," chapter "What the Prophet ﷺ Mentioned," etc., hadith no. 7330, from Amr bin Zurara with the same wording. It is also found in Kubra, chapter "Encouraging Agreement Among the People of Knowledge," hadith no. 2298, where Ziyad narrated it from Qasim with the same wording.