Sefer Yetzira was written by Avraham Avinu who was born in 1948 from creation of the world. Though many editions and formations came about after, the basic text was made by him. This is the earliest known midrash available for people nowadays and has many perushim from Rishonim like the Raavad and Ramban.
source: Rav Abraham Azulai(1685). חסד לאברהם, מעין חמישי, נהר נא . It is also written in the appendix of many editions of sefer Yetzira itself.