wer einmal lügt, dem glaubt man nicht, und wenn er auch die Wahrheit spricht

Etymology
lit. 'he who lies once, will not be believed even when he speaks the truth'.

Proverb

 * 1) it is hard to trust someone who has been caught in a lie before; once a liar, always a liar

Usage notes

 * May be shortened to, or even just.