Islam has laid stress on telling the truth and avoiding lies.
Allah the Most High said: “Surely the men who submit and the women who submit, and the believing men and the believing women, and the obeying men and the obeying women, and the truthful men and the truthful women, and the patient men and the patient women, and the humble men and the humble women, and the almsgiving men and the almsgiving women, and the fasting men and the fasting women, and the men who guard their private parts and the women who guard, and the men who remember Allah much and the women who remember - Allah has prepared for them forgiveness and a mighty reward.”
The Holy Prophet Muhammad (S) said: “Three things are the signs of hypocrisy: breach of trust, lying and breach of promise.”
He (S) also said: “He who has these three things should be counted a hypocrite, even if he performs the daily prayers, observes fasting and thinks that he is a Muslim: breach of trust, lying and breach of promise. Allah the Most High said: ‘Surely Allah does not love the treacherous;’1 ‘The curse of Allah be on him if he is one of the liars;’2 ‘And mention Ismail in the Book; surely he was truthful in (his) promise and he was an apostle, a prophet.’3”
Imam ‘Ali (a.s.) the Commander of the Faithful said: “O people, be truthful, surely Allah is with the truthful; and avoids lying, for it stands far from faith.”
He (a.s.) also said: “A servant of Allah does not taste faith unless he gives up lying earnestly or jokingly.”
Imam Muhammad al-Baqir (a.s.) said: “Lies destroy faith.”
Imam Ja’far al-Sadiq (a.s.) said: “Keep on having fear of Allah, truthfulness and trustworthiness.”
In another saying he listed lying among grave (mortal) sins; the same thing being found in an epistle of Imam ‘Ali al-Rida (a.s.) to the caliph al-Ma’mun.