"Beware of jealousy, for verily it destroys good deeds the way fire destroys wood."
Hasad (jealousy and envy) is among the most destructive emotions or 
feeling which a man may have towards his fellow human being. It causes 
him to wish evil for others and to be happy when misfortune befalls 
them. The Prophet (sallallâhû alaihi wa-sallam) warned against envy by 
comparing it to fire that completely burns the wood.
He (sallallâhû alaihi wa-sallam) said: "Beware of jealousy, for verily it destroys good deeds the way fire destroys wood." [Abû Dawûd]
Hasad is a disease of the heart and it causes impurity to the heart, 
when Allâh's Messenger (sallallâhû alaihi wa-sallam) was asked who are 
the best of people? He (sallallâhû alaihi wa-sallam) replied: "the one 
with a clean heart and truthful tongue."They asked: 'We understand a 
truthful tongue, but what does a clean heart mean?' he answered: 'It is 
the heart of one that is pious, pure, and is free of sin, 
transgressions, hatred and Hasad." [Ibn Mâjah]
Hasad is such a dangerous characteristic that Allâh revealed verses 
of the Qur'ân to be recited as a protection from the jealous,
"Say: I seek refuge in the Lord of the dawn… from the evil of envious when he envies." [Sûrah al-Fâlaq (113): 1]
At-Tirmîdhî narrated from al-Zubayr Ibn al-Awam that the Prophet 
(sallallâhû alaihi wa-sallam) said: "There has come to you the disease 
of the nations before you, jealousy and hatred. This is the 'shaver' 
(destroyer); I do not say that it shaves hair, but that it shaves 
(destroys) faith…" [(Hasan) Jamî at- Tirmîdhî (2434)]
Hasad can cause the person to indulge in disbelief because it causes 
the individual to feel that Allâh has not been fair with him; he forgets
 all the mercy and blessings which Allâh has bestowed upon him. The 
Messenger of Allâh (sallallâhû alaihi wa-sallam) said: "They are enemies
 for Allâh's bounties." They asked: "Who are they?" He said: "Those who 
envy people for what Allâh has given them of Bounty." [at-Tabarânî]
Allâh through His Absolute Wisdom has given some people more wealth, 
intelligence, beauty, strength, children, etc. than others. The 
believing Muslim should be content with what Allâh has destined for him.
Allâh says: "Allâh favored some of you over others with wealth and properties… Do they deny the favors of Allâh?" [Sûrah an-Nahl (16): 71]
And: "Do they envy men for what Allâh has given them of His Bounty?" [Sûrah an-Nisâ (4): 54]
"It is We Who portion out between them their livelihood in this 
world, and We raised some of them above others in ranks, so that some 
may employ others in their work. But the mercy of your Lord is better." [Sûrah az-Zukhruf (43): 32]
meaning mercy of Allâh is better than the convenience of the world. 
The materials of this life do not make one superior to another in 
Allâh's Judgment. True superiority lies in Taqwa (righteousness, fear of
 Allâh).
He said: "Surely, the most noble of you to Allâh is the most God-fearing." [Sûrah al-Hujurât (49): 13]
"And the Hereafter with Your Lord is (only) for those who have Taqwa." [Sûrah az-Zukhruf (43): 35]
What belongs to the transient world is of no significance before 
Allâh. The Prophet (sallallâhû alaihi wa-sallam) said: "If this world 
were worth a mosquito's wing before Allâh, He would not give a 
disbeliever a drink of water." [At-Tirmidhî]
The favors of Allâh in the world are a test; the more the favors, the
 more the tests. Al-Hasan al-Basri said: "Umar Ibn Khattâb wrote this 
letter to Abû Musa al-Ash'ari, 'Be content with your provision in this 
world, for the Most Merciful has honored some of His servants over 
others in terms of provision as a test of both. The one who has been 
given plenty is being tested to see if he will give thanks to Allâh and 
fulfill the duties which are his by virtue of his wealth…" [Ibn Hâtim]
Allâh, the Exalted, has therefore forbidden us from desiring what other have, "Do not wish for what we have favored some of you over others." [Sûrah an-Nisa (4): 32]
In order to discourage envy, the Prophet (sallallâhû alaihi 
wa-sallam) said: "Do not look to those above you. Look to those below 
you, as it will more likely remind you of Allâh's favors bestowed on 
you." [Sahîh al-Bukharî and Sahîh Muslim] On another occasion, he said: 
"If one of you looks at someone wealthier and better built than him, he 
should also look at someone of lower standard than himself." [Sahîh Muslim]