The Sefer Shem Mishmuel (page 302) cites Rabbi Berachyah in Shemos Rabbah Perek 28, posuk 1:
“The Tablets were six tefachim (handbreadths) long — in some sense, Hashem grasped two tefachim, Moshe grasped 2 tefachim and 2 tefachim bridged the gap between them.”
Shem Mishmuel then explains (pages 302 – 304):
We can sub-divide all mitzvot, and indeed, all human endeavors into three spheres: thought, speech and action. There are some Mitzvot which require a Jew to think in a particular way. For example, the first of the Ten Commandments demands belief in Hashem.