Charles Ellicott Commentary Exodus 19:3

"And Moses went up unto God, and Jehovah called unto him out of the mountain, saying, Thus shalt thou say to the house of Jacob, and tell the children of Israel:" — Exodus 19:3 (ASV)

Moses went up unto God — i.e., ascended Sinai, where he expected that God would speak with him.

The Lord called unto him out of the mountain. —While he was still on his way, as it would seem, so that he was spared the toil of the ascent. God meets us half-way when we “arise and go” to Him.