Shama-il Tirmidhi Chapter 021, Hadith Number 002 (124).
Abu Bakrah radiyallahu anhu relates that, "Rasoolullah sallallahu alaihe wasallam once said, :Must I show you a great sin, from among the greatest sins?" The sahaabah replied, "Yes O Rasoolullah sallallahu alaihe wasallam, do tell us." Rasoolullah sallallahu alaihe wasallam replied, "To ascribe a partner unto Allah. To disobey one's parents. To bare false witness, (or tell a lie). (The narrator is not sure which of the two Sayyidina Rasoolullah sallallahu alaihe wasallam had said). At that time Rasoolullah sallallahu alaihe wasallam was leaning on something. When he mentioned lies, he sat up, and because of its importance began to repeat it many times, till we began hoping that he would stop, and not repeat it so many times".
The reason for the sahaabah radiyallhu anhum wishing that Sayyidina Rasoolullah sallallahu alaihe wasallam should stop repeating it, is because of their love for Sayyidina Rasoolullah sallallhu alaihe wasallam, and there fear that he would become tired. It is enough to say a thing once. It could be possible that he might have become angry, or that in anger, Sayyidina Rasoolullah sallallahu aliahe wasallam might say something that would become a cause of loss to the ummah. Those who attend these type of religious or wordly assemblies are well aware of such situations. Sayyidina Rasoolullah sallallhu alaihe wasallam repeated it to show its importance. Lies and falsehood inevitably gets a person involved in adultery, murder and many other sins. Sayyidina Rasoolullah sallallahu alaihe wasallam also said, "When a person lies, the angels stay a distance of a mile away from him because odour it causes in person's mouth". He has also said, "A believer cannot be a liar". Sayyidina Abu Bakr Siddique radiyallahu Anhu says, "Stay away from lies, because lies keep away Imaan." (Al-Itidaal)
In this hadith the major sins are mentioned. In the shari'ah there are two types of sins. One is the minor sins, which are forgiven after performing wudu, sallah, saum, haj etc. The second is the major or greater sins. These are never forgiven without sincere taubah (repentance). Certainly if Allah wills He will forgive even the major sins through His Great Mercy. This is an exception, but the fundamental laws is that they are not forgiven without sincere taubah. The 'alama differ as to the count of major sins. Special books have been written on this subject. 'Allaamah Dha-habi has written a special treatise on this subject, wherein he has counted four hundred major sins. Allamah Ibn Hajar has also written a two volume book which has been printed in Egypt. He has mentioned all the major sins on every subject, like that of salaah, saum, zakaah, haj etc. He has counted four hundred and sixty seven sins major sins in detail. Mulla Ali Qarri has written the commentary of the Shamaa-il Tirmidhi, the common major sins as follows: To murder a person; adultery; homosexuality; the drinking of wine and other intoxicants; theft; falsely accusing a person; refusing to give evidence of a factual incident; take a false oath; unlawfully taking possession of anothers wealth or belonging; running away from war against non-believers without a valid reason; dealing in interest; stealing an orphan's possessions; taking and giving bribes; disobeying parents; cutting of relationship with one's relatives; relating a false hadith; breaking fast in Ramdhaan (without a valid reason); cheating in measures and weights; performing a fardh salaah before or after its stipulated time; avoiding paying zakaah; hitting or ill-treating a Muslim or non-Muslim with whom an agreement has been made; accusing or talk ill of the Sahabah radiyallahu anhum; backbiting especially about an aalim or a haafiz of Qur'aan; telling tales (backbite) to an oppressor; being shameless and accept the prostitution or unlawful habits of one's wife or daughter's; pimping; abstaining from Amri-bin-ma;ruf Wan-nahyi anil munkar (commanding to do good, and prohibiting from committing evil), whilst having the strength and means to do so; practising black-magic (sihr-jadu) and teaching to others; casting a spell (black-magic) on someone; learning the Qur'aan and forget it; burning a living thing without a valid reason; giving up hope of attending Allah's mercy; having a fear of His punishment; a woman disobeying her husband and without a valid refused to fulfil his desires.
Mulla Ali Qaari has given the above examples of major sins in the Mazaahiri Haq, translation and commentary on Mishkaat a special chapter has been written in the beginning, wherein similar sins have been mentioned. It has also been stated there in that to ascribe a partner on to Allah, in whatever form, e.g. making a partner in praying to him; in seeking assistance; in knowledge; in his divine power and authority in creating; in calling (someone not present) for help; in naming; slaughtering; in offering a vow; or the handling over all such in matters to people instead of Allah; etc. The following are also included in the kitaab: To make an intention to persist in sinning; to consume intoxicants; marry one's mahram; gamble; keep friendship with non-Muslim; not to take part in Jihaad, whilst having the power to do so; eat the meat of a dead animal; have faith in an astrologer, and forecasts of soothsayers; criticise Sayyidina Rasulullah sallallahu alaihe wasallam and the malla-ikah, and to deny his (Sayyidina Rasulullah sallallahu alaihe wasallam's) prophethood and their (malaa-ikah's) existence; criticise the Sahabah radiyallahu anhum; cause a quarrel and fight between husband and wife; be wasteful; create rebellion, intrigue or mischief; appear mude before others (besides one's wife, or husband); be niggardly and stingy; not cleansing one's self from urine and sperm (i.e. if these fall onto the clothing and body and are not washed); disbelieve and refute taqdder (divine fore-ordainment); lower the trouser or lungi below the ankles in pride; lament (no-hah) a persons death; begin or invent a bad mode, thing or manner; be ungrateful to a benefactor; call a Muslim a disbeliever; have intercourse with a menstruating woman; become happy when food becomes scarce; commit a sexual act with an animal; look at a man with lust; peep or spy in another's house; degrade the Ulama or the Huffaaz of the Qur'aan; fail to treat all wives equally if one has more than wife; be unfaithful to amir or leader etc.
The major sins also have their stages. That is why in the above Hadith only the main ones among the major sins are mentioned according to their circumstances. The Ulama have written that to keep on committing a (minor) sin, results in it becoming a major sin, and by repenting sincerely a major sin is forgiven. A sincere taubah is that, one should make a firm resolution that one shall not commit that sin again.