It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that he said: "Whoever earns what is lawful, withholding the zakah makes it impure; and whoever earns what is unlawful, (paying) the zakah does not make it pure."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9562