Jealousy is one of the most destructive of evil traits but, unfortunately, so many of us are afflicted with it without even knowing. The Messenger of Allah (صلى الله عليه وسلم) has spoken very harshly concerning this evil trait. He (صلى الله عليه وسلم) said,
“Iman and jealousy never come together in the heart of a servant.” 
We have mentioned a few more ahadith on this issue at the end of this article. It is extremely important that we understand the wide scope of jealousy so we may refrain from it.
Definition of Jealousy
تمني زوال نعمة الغير
To wish or hope another person loses his blessing.
Blessing is general, whether of a worldly nature or a religious one. Religious blessings include Islamic knowledge, ebadah, dawah and piety etc. Worldly blessings include things like health, strength, beauty, skills, wealth and position etc.
Jealousy or hoping that another loses a blessing includes a number of scenarios:
Person A wishes B loses a blessing even though A does not care to acquire that blessing. This is the worst form of jealousy. It clearly points to the extent of malice filling a person’s heart.
Person A wishes B loses a blessing and that it is transferred to A. For instance he wishes B loses his wife and he marries her.
Person A wishes to receive a similar blessing that B possess. Whether A receives this blessing or not, he hopes that B losses it. For instance B has a car. A wishes that he also gets a similar car. Whether A gets a similar car or for some reason is unable to get it, he wants that B loses his car.
Person A hopes that B remains in a sad or bad state. For instance, B is poor or ignorant or deviated or oppressed or grieved or weak and A hopes that B remains in any of those or any other bad conditions.
To wish any harm for a Muslim. 
N.B. Jealousy is an action of the heart. Simply entertaining such thoughts is jealousy and sinful. If one acts on it by backbiting, slandering, defaming or physically harming the other person then that is a second sin.
All of these cases come under the ambit of jealousy as explained by Imam Ghazali and Ibn Qaiyyim Jawziyyah. It is incumbent on us to look into our hearts and see if we have any such feelings towards any of our brothers or sisters. If we do, we must repent and ask Allah to forgive us. Then we must immediately start loving for all our Muslim brothers and sisters what we love for ourselves. We should make dua of prosperity and increase in all forms of blessings for those whom we previously had malice towards. Without this, we lose our Iman as the hadith above warned. The Messenger of Allah (صلى الله عليه وسلم) also categorically said that, “None of you are believers until you love for your brother what you love for yourself.”
He (صلى الله عليه وسلم) also warned, “The disease of the nations before you is creeping towards you: Jealousy and hatred, it is the Haliqah (shaver). I do not speak of what shaves the hair, but what uproots the religion.” 
He (صلى الله عليه وسلم) also cautioned, “Beware of jealousy, because it consumes good-deeds like how fire consumes dry wood.” 
Ibn Qaiyyim says that the jealous person is a soldier of Iblis, rather one of his vicegerents (khalifah) and substitutes.
May Allah allow us to purify our hearts from all forms of malice and jealousy and may He allow us to meet Him on the Day of Judgment with a sound and pure heart. Ameen.