To speak ill of anyone is to speak ill of yourself.
More Afghan Proverbs:
The ungrateful son is like a wart on his father's face; to leave it there is unsightly, to cut it off is painful.Afghan
The seeker is the finder.
Afghan
Cats don't catch mice to please God.
Afghan
The wood is burnt, but the ashes are a nuisance.
Afghan
There are twenty-five uncaught sparrows for a penny.
Afghan
First food, then religion.
Afghan