7 Major Sins in Islam: What Are They?
Question

What are the seven major sins in Islam and what are their punishments?
Praise be to Allah.Hadith about major sins in Islam
The Prophet (blessings and peace of Allah be upon him) explained the seven sins that doom a person to Hell i n a sahih hadieth that was narrated by the two Shaykhs, al-Bukhari and Muslim, in as-Sahihayn, in which it was narrated that the Prophet (blessings and peace of Allah be upon him) said: “Avoid the seven sins that doom a person to Hell.” We said: What are they, O Messenger of Allah? He said: “Associating others with Allah (shirk); witchcraft; killing a soul whom Allah has forbidden us to kill, except in cases dictated by Islamic law; consuming orphans’ wealth; consuming riba; fleeing from the battlefield; and slandering chaste, innocent women.”  
7 major sins in Islam
1. Shirk
The greatest of these sins is shirk (associating others with Allah), which leads to doom with no hope of redemption, and if a person dies in this state he will abide in Hell for all eternity. Allah, may He be exalted, says (interpretation of the meaning):
“Verily, whosoever sets up partners in worship with Allah, then Allah has forbidden Paradise for him, and the Fire will be his abode. And for the Zalimun (polytheists and wrongdoers) there are no helpers.” [al-Maidah 5:72]
“And indeed it has been revealed to you (O Muhammad (blessings and peace of Allah be upon him)), as it was to those (Allah’s messengers) before you: "If you join others in worship with Allah, (then) surely (all) your deeds will be in vain, and you will certainly be among the losers.” [az-Zumar 39:65]
“It is not for the Mushrikun (polytheists, idolaters, pagans, disbelievers in the Oneness of Allah), to maintain the Mosques of Allah (i.e. to pray and worship Allah therein, to look after their cleanliness and their building, etc.), while they witness against their own selves of disbelief. The works of such are in vain and in Fire shall they abide.” [at-Tawbah 9:17] 
2. Witchcraft 
Witchcraft involves shirk because it is worshipping the jinn and is seeking the help of the jinn to misguide people. 
The practitioner of witchcraft is the one who deals with that which causes harm to people by means of the jinn and worshipping them instead of Allah. Sometimes he does things that harm them of words, deeds and blowing on knots; sometimes he does that by causing illusions to such an extent that a thing may be seen as something other than what it really is, as Allah, may He be glorified, said concerning the sorcerers of Pharaoh (interpretation of the meaning):
“…their ropes and their sticks, by their magic, appeared to him as though they moved fast.” [Ta-Ha 20:66]
“So when they threw, they bewitched the eyes of the people, and struck terror into them, and they displayed a great magic.” [al-A'raf 7:116] 
Sometimes the practitioner of witchcraft does things to harm people by means of the jinn and worshipping them instead of Allah, by means of words, deeds and blowing on knots, as Allah, may He be exalted, says (interpretation of the meaning): “And from the evil of the witches when they blow in the knots.” [al-Falaq 113:4]
And sometimes they do that by creating illusions, so that they cause things to be seen as something other than they really are. So a rope or a stick may be seen as a snake, or a rock may be seen as an egg, or a person may be seen as something other than he really is, and so on. Such a person is one of the disbelievers and if it is proven before the judge, it is obligatory for the ruler of the Muslims to execute the practitioner of witchcraft. 
It was proven from Amir al-Muminin ‘Umar (may Allah be pleased with him) that he wrote to his governors in greater Syria and elsewhere, instructing them to execute every practitioner of witchcraft, male or female, because of the great extent of their evil and the danger they posed. 
3. Killing 
Killing a soul whom Allah has forbidden us to kill. This is a serious crime of which Allah says (interpretation of the meaning):
“And whoever kills a believer intentionally, his recompense is Hell to abide therein, and the Wrath and the Curse of Allah are upon him, and a great punishment is prepared for him.” [an-Nisa 4:93] 
Killing a soul, or murder, is one of the most serious of crimes, but it is a major sin of a lesser degree than shirk (associating others with Allah). It is a major sin like fornication/adultery, theft and so on. The one who does that is not a disbeliever unless he regards it as permissible to do that. Hence Allah, may He be glorified, says concerning him:
“And whoever kills a believer intentionally, his recompense is Hell to abide therein, and the Wrath and the Curse of Allah are upon him, and a great punishment is prepared for him.” [an-Nisa 4:93]
He will abide in that punishment forever if he regards it as permissible; if he does not regard it as permissible, he will abide therein for a long time, but it will come to an end. The stay of the sinners (among the Muslims) in Hell will be lengthy, but it will have an end. As for the disbelievers, their stay will have no end. The Prophet (blessings and peace of Allah be upon him) said: “Avoid the seven sins that doom a person to Hell.” We said: What are they, O Messenger of Allah? He said: “Associating others with Allah (shirk); witchcraft; killing a soul whom Allah has forbidden us to kill, except in cases dictated by Islamic law…” Cases dictated by Islamic law include execution of the previously married adulterer, by stoning; execution of a person who kills another, when both are of equal standing; execution in cases where there is a legitimate reason for executing the criminal, such as banditry, in which the criminal tries to harm travellers by seizing their wealth on the road, or beating them or killing them – he is to be executed because of his great evil. 
4. Consuming riba 
Consuming riba – this refers to dealing with riba which Allah has prohibited. Allah, may He be glorified and exalted, says concerning it (interpretation of the meaning):
“Allah has permitted trading and forbidden Riba.” [al-Baqarah 2:275]
“O you who believe! Fear Allah and give up what remains (due to you) from Riba (from now onward) if you are (really) believers. And if you do not do it, then take a notice of war from Allah and His Messenger.” [al-Baqarah 2:278]
Consuming riba is a major sin which it is essential to avoid. 
Types of riba
Riba is of various kinds, riba nasiah and riba fadl: 
·        Riba fadl is, for example, selling one dirham for two dirhams, or one sa‘ of wheat for two sa‘s of wheat, or one sa‘ of rice for two sa‘s of rice. This is riba fadl, which means selling something for more of the same type or commodity. This is not permissible. 
·        Riba an-nasiah is, for example, selling a sa‘ of wheat for two sa‘s of barley to be paid later, after one or two days, i.e., the price is not to be taken until after the sitting in which the deal is done. This is riba nasiah. It may be selling one hundred dollars for one hundred pounds, or for ten pounds, to be paid at another time, and it is not taken in that sitting. This is called riba an-nasiah, and it is one of the gravest of major sins. 
5. Consuming orphans’ wealth 
The orphan is the one whose father has died when he is small and has not yet reached puberty. What is required in the case of orphans is to treat them kindly, protect their wealth, make it grow and handle it with proper care. The one who spoils the orphan’s wealth and consumes it unlawfully is subject to a stern warning, because the orphan is weak and he has transgressed against him and consumed his wealth. This stern warning is addressed to him, but he is not a disbeliever; rather he is a sinner, so long as he does not regard that as permissible. 
6. Fleeing from the battlefield 
When the Muslims meet the disbelievers in battle, the one who flees and deserts his brothers on the day of battle, when the disbelievers march against the Muslims or the Muslims march against the disbelievers – the one who flees and deserts his brothers is subject to this stern warning (interpretation of the meaning): “unless it be a stratagem of war, or to retreat to a troop (of his own)” [al-Anfal 8:16] that is, unless he is recruiting to prepare himself, bring his weapon and put on his armour in preparation for fighting, which does not do any harm, or he is moving from one detachment to another, or from one rank to another, or from one group to another, as part of a plan against the enemy. 
7. Slandering chaste, innocent believing women 
The one who slanders chaste women , which means accusing them of zina (fornication or adultery) – by saying: So and So committed zina, or So and So called someone to commit zina, but he is lying – this is one of the seven sins that doom one to Hell and for which one deserves to be given eighty lashes, as Allah, may He be exalted, says (interpretation of the meaning): “And those who accuse chaste women, and produce not four witnesses, flog them with eighty stripes” [an-Nur 24:4]. Slandering chaste women is a major sin, as is slandering chaste men, but because such accusations are usually made against women, the text speaks of women. Nevertheless, if someone slanders a chaste man, and says that he committed zina, he has to produce four witnesses or else be given eighty lashes. 
These are the seven sins that doom a person to Hell, because they are so evil. We ask Allah to keep us safe and sound.” (Adapted from the website of Shaykh Ibn Baz)  
And Allah knows best.