You cannot get blood from a stone

You cannot get blood from a stone.

 and You cannot get blood from a turnip.

Prov. You cannot get help from an uncharitable person or money from someone who has none. Jerry and James spent two hours trying to convince the old miser to contribute to the children's hospital; finally, James turned to Jerry and said in disgust, "This is hopeless. We can't get blood from a stone." The government can't increase taxes any further—nobody has the money! You can't get blood from a turnip.

See also: blood, cannot, get, stone