These words are not in the Old Testament at all. This is one of those rare occasions when something is added by the Spirit in the New Testament.
The God of Mount Sinai who revealed himself to Moses is not a different God to the God and Father of our Lord Jesus Christ. It is the liberal theologians who want to separate the two Testaments as if they described two completely different beings. The marvel of grace is that the same God whose sternness and strictness and exalted holiness, caused such terror on Sinai, is the very same one who invites sinners to draw near, and who tells them to come with confidence in his clemency. This is not pictured by a king who has left his throne and put on casual clothes and come to the local hostelry to sip a drink with his citizens. It is pictured by a sovereign king who has already proven his power to dispense life and death, and yet who invites the worst offenders in his kingdom to overcome their fears and to draw near to his throne for mercy, the same throne from which justice proceeds to destroy his enemies. Yes, Christ did eat and drink with publicans and sinners, and this gave reassurance to sinners, but he never abandoned his throne, and his glory that was hidden while he was on earth is now taken up again with full force. Is it surprising that we have difficulty putting together the majesty of the Son of God with his humiliation on the cross of Calvary? But it is our duty to hold both of these things at once.