Pennies from heaven
Money acquired without effort or risk. The phrase is applied to any unexpected benefits, but especially financial ones.
The first time that the expression "Pennies from heaven" came into the public consciousness was on the release of the 1936 film, starring Bing Crosby.
It wasn't coined by the film's writers though, having been used in print a few years earlier, in Abraham Burstein's Ghetto Messenger, 1928.