Honesty is like an icicle; if once it melts that is the end of it

Don't measure your neighbor's honesty by your own.

If you truly want honesty, don't ask questions you don't really want the answer to

It pays to be honest, but it's slow pay

Lie to a liar, for lies are his coin; Steal from a thief, for that is easy; lay a trap for a trickster and catch him at first attempt, but beware of an honest man

It never rains, but it pours