Parshat Tazria-Metzora

The Psalmist compares slanderous talk to "Sharp arrows of the warrior, coals of broom" (Psalms 120:4).

All other weapons smite from close quarters, while the arrow smites from the distance. So is it with slander: it is spoken in Rome and kills in Syria.

All other coals, when extinguished, are extinguished without and within; but coals of broom are still burning within when they are extinguished without. So is it with words of slander: even after it seems that their effects have been put out, they continue to smolder within those who heard them. It once happened that a broom tree was set on fire and it burned eighteen months -- winter, summer and winter.

(Midrash Rabbah)