kill the goose that lays the golden eggs
Destroy a source of riches through stupidity or greed, as in If he never gives his loyal customers a break on some items in his store, he'll kill the goose that lays the golden eggs. This expression, already a proverb in the late 1400s, alludes to Aesop's fable about a farmer whose goose lays one golden egg a day, and who kills the goose in the mistaken belief that he'll get all the eggs at once.