Midrasch zu Tehillim 62:11

Rabbi Jehudah said: When a man speaks with his companion, he || hears the sound of his voice, but he does not see any light with it; the Israelites heard the voice of the Holy One, blessed be He, on Mount Sinai, and saw the voice going forth from the mouth of the Almighty in the lightning and the thunder, as it is said, "And all the people saw the thunderings and the lightnings" (Ex. 20:18). All the precepts which are in the Torah number 611, and two, which the Holy One, blessed be He, spake, as it is said, "God has spoken once, two have I heard thus" (Ps. 62:11).