Lying, where it is not allowed (and there are only a few exceptions where it is allowed) is a heinous sin and puts one in grave danger of corruption, hypocrisy and even kufr.  It is a "slippery slope" to all manner of corruption and a character flaw from which it is difficult to return.  Fellow Muslims, search within yourself and rid yourself of all forms of lying, deception and falsehood.  Make sure that truthfulness is victorious over falsehood in your heart at all times.  Allah does not guide one who is a liar.  Allah said:

{Wa qaala rajulun mu'minun min aali fir'auna yaktumu imaanahu a taqtuloona rajulan an yaqoola rabiyya Allah wa qad jaa'akum bil-bayyinaati min rabbikum?  Wa in yaku kaadhiban fa 'alaihi kadhibuhu wa in yakun saadiqan yusibkum ba'dhu alladhiy ya'idukum inna Allaha laa yahdiy man huwa musrifun kadh-dhaabun.}
{And a believing man from Pharaoh's people who had been hiding his belief said: Will you kill a man simply because he says my Lord is Allah, though he has brought you clear proofs from your Lord?  If he is lying, his lie is upon him but if he is telling the truth, you will be afflicted by some of what he is warning you about.  Verily, Allah does not guide one who is given to excess and lying.}  Ghaafir:28

One of the worst forms of lying in which a Muslim can engage is to be phony about his/her Islam.  Al-Hassan Al-Basri, an early taabi'i, was a famous waa'idh (preacher) who could bring large numbers of people to tears in moments.  No one doubted his piety buy himself.  He used to go to his home every night after doing whatever good Allah knows for Islam and tremble in fear that his actions may not measure up to his words.  He did this in true contemplation and understanding of Allah's statement:

{Yaa ayyuhaa alladhina aamanoo lima taquloona maa laa taf'aloona? (2)  Kabura maqtan 'inda Allahi an taqooloo maa laa taf'aloona. (3)}
{O, you who believe, why do you say that which you do not do? (2) It is very hateful to Allah that you would say that which you do not do.}  As-Saff:2-3

The worst forms of lying is to tell lies (or speak without knowledge) about Allah, His Messenger and His Message (Islam).  The source of this knowledge is Allah (via the Qur'an) and His Messenger (via the athentic hadith).  There is no other route to this knowledge and the knowledge was sent by Allah is is being preserved by Allah.  Al-hamdu lillahi!  The knowledge is there for the seeking until yaumil Qiyaama.  The previous nations did not get this.  There were put in charge of preserving their messages and they lost them all.  Allah himself took charge of preserving the message of Islam - even if that was actually carried out by human means.  Allah said:

{Al-Haqqu min rabbika fa laa takun min al-mumtareena (60) Fa man haajjaka feehi min ba'di maa jaa'aka min al-'ilm fa qul ta'aalau nad'u abnaa'anaa wa abnaa'akum wa nisaa'anaa wa nisaa'akum wa anfusanaa wa anfusakum thumma nabtahil fa naj'al la'nata Allahi 'alaa al-kaadhibeena. (61)}
{The truth is from your Lord so do not be among those who doubt. (60) Then, if anyone disputes with you about it after that knowledge which has come to you, simply say to them:  Come, let us call our sons and your sons, our women and your women, ourselves and yourselves and let us make a solemn prayer and invoke Allah's curse upon whoever is lying. (61)}  Aal 'Imraan:60-61

This includes lying about Allah's laws (the shari'a) and speaking about what is halal and what is haram without authentic knowledge from Allah and/or from His Messenger (sas).  Allah said:

{Wa laa taquloo limaa tasifu alsinatukum al-kadhiba hadhaa halaalun wa hadhaa haraamun li taftaroo 'alaa Allahi al-kadhiba inna alladhina yaftaroona 'alaa Allahi al-kadhiba laa yuflihoon.}
{Do not say about that which your lying tongues describe that this is lawful and that is not lawful, that you may forge a lie upon Allah.  Verily, those who forge lies upon Allah never succeed.}  An-Nahl:116

{Qul inna alladhina yaftaroona 'alaa Allahi al-kadhiba laa yuflihoona (69) Mataa'un fiy ad-dunyaa thumma ilainaa marji'uhum thumma nudheeqahum al-'adhaaba ash-shadeeda bi maa kaanoo yakfuroon. (70)}
{Say, verily those who forge lies upon Allah never succeed. (69) Some provision in this world but unto Us is their return and then we will make them taste the severe punishment for the kufr which they used to commit. (70)}  Yunus:69-70

In addition to lying about Allah being one of the greatest sins and a cause of punishment in the hereafter, in the above verses, we see that:

1. Material gain is one of the motives for which people commit it.
2. People who commit this will never succeed.
3. They may get "goods" (mataa') in this life, but will only eventually return to Allah and be serverely punished.
4. What they did was kufr.

Lying about Allah's creation is also a major sin and a source of many kinds of corruption, division and strife.

"Inna min a'dham al-firaa an yud'aa ar-rajulu ilaa ghairi abeehi, au yuriya 'ainahu maa lam tara, au yaqoola 'alaa rasooli Allahi maa lam yaqul."
"The most heinous of forgeries as for a man to be ascribed to other than his true father, to claim that his eye saw what it did not see and to attribute to Allah's Messenger (sas) something which he did not say." Al-Bukhari

Adh-dhann

"Iyaakum wa adh-dhanni fa inna adh-dhanna akdhaba al-hadeeth."
"Stay away from assumption, for assumption is the most lying of all discourse."  Muslim & Al-Bukhari

transmitting everything you hear

"Kafaa bil-mar'i kadhiban an yuhadditha bi kulli maa sami'a."
"A person will be filled with lies simply by narrating everything he hears."  Muslim

Lying leads to further corruption and even hell-fire

"Inna as-sidqa yahdiy ilaa al-birri wa inna al-birra yahdiy ilaa al-jannati.  Wa inna ar-rajula la yasduqu hattaa yuktaba 'inda Allahi siddeeqan.  Wa inna al-kadhiba yaydiy ilaa al-fujoori wa inna al-fujoora yahdiy ilaa an-naari. Wa inna ar-rajula la yakdhibu hattaa yuktaba 'inda Allahi kadh-dhaaban."
"Truthfulness guide to righteousness and righteousness guides to Paradise.  Verily, a man will be truthful until he is recorded with Allah as ever-believing.  And lying guides to moral corruption and moral corruption guides to the fire.  Verily, a man will continue to lie until he is recorded with Allah as a complete liar."  Muslim & Al-Bukhari

A munaafiq is one whose apparent external reality is Islam, belief, practice of Islam, etc. but whose inner reality is disbelief and eveil intent toward Islam and the Muslims.  It is NOT simply "hypocrisy" (which covers many things which are not "nifaq") and that is a very inadequate and misleading of the term.  The character flaw possessed by a munaafiq is called "nifaaq". Nifaaq negates Islam even if one is "practicing" in the apparent

"Aayatu al-munaafiqi thalaathun wa in sallaa wa saama wa za'ama annahu Muslim:  idhaa haddatha kadhiba, wa idhaa wa'ada akhlafa wa idhaa 'tumina khaan."
"The signs of a munaafiq are three - even if he prays, fasts and claims/believes he is a Muslim:  when he speaks he lies, when he promises he breaks his promise and when he is entrusted he betrays the trust."  Muslim & Al-Bukhari

These hadith clearly tell us that lying or truthfulness are character traits.  They are habit-forming, are "slippery slopes" and are not something that any human can turn on and off at will.  One who is a liar to people will also be a liar to Allah and vice versa.  That is why a Muslim must take utmost precaution and steps against ANY form of lying, deception or falsehood whether with/about Allah or with/about people.

Malik ibn Deenaar said:  "As-sidqu wa al-kadhibu ya'tarikaani fiy al-qalbi hattaa yukhrija ahaduhumaa saahibahu."  "Truthfulness and lying are in combat in the heart until one of them expels the other."

Lying is one of the weaknesses of the nafs and one of the wrong things to which it becomes attracted.

Luqman is reported to have said to his son:  "Yaa bunayya iyaaka wa al-kadhibi fa innahu shahiyyun ka lahmi al-'usfoori 'ammaa qaleelun yaqlaaho saahibuhu."  "Dear son, beware of lying for it is surely attractive like the meat of a quail which its owner is just about to pluck off and eat."

It is never beneficial, even if it appears to be so in the short run.

Ash-Sha'biy said:  "Iyaaka wa al-kadhiba haitha taraa annahu yanfa'uka fa huwa yadhurruka."  "Beware of lying.  Whenever you think it will benefit you, it only harms you."