Albert Barnes 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 - This seems to imply that the voice was heard by Moses as he was ascending the mount.

House of Jacob - This expression does not occur elsewhere in the Pentateuch. It has a special fitness here, referring doubtless to the special promises made to the Patriarch.