Salvation is the work that is being spoken of in verse 41.
39 And by him all that believe are justified from all things, from which ye could not be justified by the law of Moses. 40 Beware therefore, ***lest that come upon you***, which is spoken of in the prophets; 41 Behold, ye despisers, and wonder, and perish: for ***I work a work in your days, a work which ye shall in no wise believe***, though a man declare it unto you.
Salvation is the work that is being spoken of in verse 41. And the unbelief thereof is what verse 41 is referring to. This was taken from Habakkuk 1.
5 Behold ye among the heathen, and regard, and wonder marvelously: for I will work a work in your days which ye will not believe, though it be told you. 6 For, lo, I raise up the Chaldeans, that bitter and hasty nation, which shall march through the breadth of the land, ***to possess the dwellingplaces that are not their’s***.
The work being spoken of in these verses are the works Jesus worked while it was Day, therefore when night time came (judgment day) God’s worked ceased (no man can work).
Psalms 119:25-32 (KJV) My soul cleaveth unto the dust: quicken thou me according to thy word.
I have declared my ways, and thou heardest me: teach me thy statutes.
Make me to understand the way of thy precepts: so shall I talk of thy wondrous works.
My soul melteth for heaviness: strengthen thou me according unto thy word.
Remove from me the way of lying: and grant me thy law graciously.
I have chosen the way of truth: thy judgments have I laid before me.
I have stuck unto thy testimonies: O LORD, put me not to shame.
I will run the way of thy commandments, when thou shalt enlarge my heart.