R. Isaac said: It was only necessary to write the Torah from the words (in Exod. 12:2): THIS MONTH SHALL BE FOR YOU. Why did he write from the words (in Gen. 1:1): IN THE BEGINNING? To make known his mighty power. Thus it is stated (in Ps. 111:6): HE HAS DECLARED TO HIS PEOPLE THE POWER OF HIS WORKS IN GIVING THEM THE HERITAGE OF THE GENTILES.
Why the Torah Did Not Start With Its First Commandment
Midrash Tanchuma Buber, Bereshit 11:1