What causes jealousy?
Muslim scholar, Ghazali mentioned seven causes for jealousy: Enmity and hostility (towards others); excessive self-satisfaction; vanity; expressing amazement at whatever others have; fearful of losing own possessions; craving for high offices and positions; and having an evil soul. Ghazali also referred to this point that jealousy is basically rooted in the heart and therefore is a characteristic of the heart.
Sometimes this characteristic shows itself through tongue or behavior which in this case the human being is committing sins and must ask for forgiveness. But sometimes jealousy stays just in the heart which in this case if the human being finds it inappropriate then must consider it bad and fight it 
 Ihyaul Oloom-e-Deen, Mohammad Ghazali, Vol. 3, page 204.
Description of pious people
The signs of knowledgeable people according to Imam Baqir (A.S)
The Mercy of Allah
Different Types of Heart