cash cow
A business or investment that generates a large or consistent profit. When Bob purchased stock in that software company 10 years ago, he never expected it to become such a cash cow. Now that it has quadrupled in value, he'll be able to retire early.
a cash cow
If you call someone or something a cash cow, you mean that it makes a lot of money. This rather unglamorous property is a cash cow, generating about $700 million a year in advertising revenue. A player like Rooney is a potential cash cow to dozens. Note: The reference is to something that produces money as freely as a cow produces milk.