manna from heaven

manna from heaven (heaven sent; an unexpected gift, help, or comfort; something that you need which you get when you are not expecting to get it; something very good that you did not expect)

Did you see this raise I got today? It’s manna from heaven!
The arrival of the rescue team was like manna from heaven to the injured climber. (McGraw-Hill Dictionary of American Idioms and Phrasal Verbs)
The offer of a new job just as she had been fired was manna from heaven to Joan. (McGraw-Hill Dictionary of American Idioms and Phrasal Verbs)
I had been unemployed for two years, so when somebody phoned me up and offered me a permanent job it was like manna from heaven. (Cambridge Idioms Dictionary)
Employees are enjoying manna from heaven in the form of year-end bonuses. (Cambridge Dictionary of American Idioms)

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Origin
In the Bible, manna was a type of bread which God gave to the Israelites when they needed food.