penny saved is a penny earned, a

What one does not spend, one will have. This maxim for thrift is so familiar that it often appears in shortened form, as in Although they can afford to buy a house right now, they're putting it off, on the principle of “a penny saved.” It appeared in slightly different form in George Herbert's Outlandish Proverbs (1640). Whether or not it originally suggested that savings earn interest is not known.