Islam lays great emphasis on telling the truth and being truthful and it considers lying to be a wicked act. In many cases, lying has been described as being more evil than drinking wine. In spite of lying being a wicked act, if not telling a lies causes a greater harm or evil e.g. murder of an innocent person or attack of the enemy on Islamic territory or enmity among believers, lying becomes permissible. In fact, if a person can do Tawriyah [translated here as deliberate ambiguity] to escape telling a lie, it is necessary to do Tawriyah.
Islamic teachings lay great emphasis on telling the truth and being truthful. Imam Sadiq (a.s.) says: "Do not look at people's prolonged Roku (bending) and Sojud (prostration) since it is likely that it may have become their habit to prolong them so much that if they do not do so, they start feeling uneasy and uncomfortable; instead look at how far they are truthful and honest in their conducts." In other words, it is truthfulness and honesty with which we can judge people.
It has been reported from Imam Baqir (a.s) that he said: "God, the Exalted, has appointed locks (barriers) for evils and indecency. The key with which these locks are unlocked is wine (because the human intelligence prevents a person from committing evil but alcoholic drinks paralyze the intellect). Then the Imam further added: "Lying is worse than wine."
One of the big harms of telling a lie is that it destroys other's confidence and trust in you. As we know that the most important asset of a society is mutual trust and general confidence and the main thing that leads to the destruction of this asset is to lie to others, commit treachery and fraud. In fact, this is one of the main reasons behind the extraordinary importance of telling the truth and not telling a lie in Islamic teachings. However, in spite of the importance of telling the truth, there may arise a situation or a circumstance where there is a pressing (urgent) need for telling a lie, in which case one is allowed to tell a lie to the necessary extent insofar as the urgent situation exists. An urgent situation is where one sees that if one does not commit an act (lying), he will fall into a greater haram. Therefore, it is permissible for such a person to tell a lie to prevent greater evil or harms such as murder of an innocent person or attack of the enemy on Islamic territory or enmity among Muslim brothers and any similar cases which are more important than the evilness of telling a lie.
It should be noted that although according to Shari'ah law lying is permissible in urgent situations to prevent a great and unbearable harm, if there is a way to repel a harm, he must adopt it. One of the legal ways and permissible escape which one can use is "Tawriyah" or equivocation or indirectness in speech [translated here as deliberate ambiguity]. It means a word which has two implications, one apparent and another hidden or implied. It is when man intends to convey the first meaning whereas the other person takes it to be other.
Deliberate ambiguity is regarded as a legitimate solution for avoiding difficult situations that a person may find himself in when someone asks him about something, and he does not want to tell the truth on the one hand, and does not want to lie, on the other. If that is needed to serve some legitimate shar’i interest that outweighs the concern about misleading the person to whom you are speaking, or it is needed for a reason that cannot be achieved without lying, then there is nothing wrong with using deliberate ambiguity as an acceptable alternative.
When there is pressing need to tell a lie, it does not make a difference whether the harm is directed to the person himself or to other believers.
Imam Reza (a.s.) said: Verily, one who tells the truth about his Muslim brother, [the truth] with which he causes him to get into a trouble, he is one of liars and he who tells a lie to repel a harm from him [Muslim brother], God sees him as one of the truth tellers."
It has been quoted from Imam Sadiq (a.s.): "Every lie will have to be accounted for in the Day of Judgment except in three instances: a lie which is used as a war technique of strategy, a lie which is resorted to make peace between two quarrelling persons and a promise given to ones family."
The Holy Prophet (s) willed to Ali (a.s.) saying to him: "Verily, God loves a lie which serves an interest and hates the truth which entails an evil."
It should be noted that when one is required to lie it is necessary for him to adhere to the amount necessary for creating peace. Therefore, he should not exceed the limit because exceeding the limits will cause him to become a liar. Imam Sadiq (a.s.) says: "A person who amends the relations between two person is not a liar."
What is inferred from the above narration is that a person is not allowed to tell a lie unnecessarily and excessively under the pretext of reconciling between two people or bringing peace and reform to society. One should stick to the amount necessary.
 - Safinatul Behar, root word "Sidq"; Al-Kafi, vol.2, pg. 104, Dar al-Kutub al-Islamiyah, Tehran, 1365 (1986).
 - Usul-e Kafi, vol.2, pg.339.
ان اللَّه عز و جل جعل للشر اقفالا، و جعل مفاتیح تلک الاقفال الشراب، و الکذب شر من الشراب.
 -Hurr Amili, Wasail al-Shi'ah, vol.12, pg. 255, hadith 16238.
 - Kulayni, Al-Kafi, vol.2, pg. 342, hadith 18. The Arabic version of the hadith is as such:
قَالَ سَمِعْتُ أَبَا عَبْدِ اللَّهِ ع یَقُولُ کُلُّ کَذِبٍ مَسْئُولٌ عَنْهُ صَاحِبُهُ یَوْماً إِلَّا کَذِباً فِی ثَلَاثَةٍ رَجُلٌ کَائِدٌ فِی حَرْبِهِ فَهُوَ مَوْضُوعٌ عَنْهُ أَوْ رَجُلٌ أَصْلَحَ بَیْنَ اثْنَیْنِ یَلْقَى هَذَا بِغَیْرِ مَا یَلْقَى بِهِ هَذَا یُرِیدُ بِذَلِکَ الْإِصْلَاحَ مَا بَیْنَهُمَا أَوْ رَجُلٌ وَعَدَ أَهْلَهُ شَیْئاً وَ هُوَ لَا یُرِیدُ أَنْ یُتِمَّ لَهُمْ.
 - Hurr Amili, Wasail al-Shi'ah, vol.12, pg.252, hadith 16229. ان الله احب الکذب فی الصلاح و ابغض الصدق فىالفساد.
 - Kulayni, Al-Kafi, vol.2, pg. 210, hadith 7 المصلح لیس بکذاب
 - See: Tehrani, Mujtaba, Balagh Website.