kill the goose that lays the golden egg(s) destroy a reliable and valuable source of income.

☞ One of Aesop's fables tells the tale of a man who owned a miraculous goose that laid eggs of gold. However, he grew dissatisfied with its production of just one egg a day and killed it in the deluded expectation of finding a large quantity of gold inside it. For brevity's sake, the expression is often shortened (presumably by those unaware of its origin) to kill the golden goose.

2012The Age (Melbourne) Fortescue chief executive Neville Power said Australia was at risk of 'killing the goose that lays the golden egg' from Labor's focus on the redistribution of wealth.