When did Benjamin Franklin say a penny saved is a penny earned?

He never did. Although "A penny saved is a penny earned" is attributed to Franklin, there is no reference anywhere to him have said or written this. The closest approximation is "A penny saved is twopence dear; a pin a day 's a groat a year," published in 1736 in Hints to those that would be Rich.

The closest of any quote to "A penny saved is a penny earned is from Henry Fielding's play The Miser (act iii, Scene 12, line 1), "Penny saved is a penny got."