Just as truthfulness is the very cornerstone of the upright person’s character and the springboard for his virtuousness, falsehood, its opposite, is the foundation of a person’s depravity and the launch pad for his wickedness. Just as the truthfulness of a person starts from within - that is, it is a reflection of a state of true faith - a person’s dishonesty, lying and deceit is also a reflection of the inner state. This is why God mentions truthfulness as being the opposite of hypocrisy:
“That God may reward the truthful for their truthfulness, and punish the hypocrites if He wills, or turn mercifully towards them...” (Quran 33:24)
…and why He mentions sincerity as a mark of truthfulness.
“So that God might recompense the truthful for their truthfulness….”
Little wonder then that the most righteous and truthful of people, the Prophets of God and their true followers, were not belied, denounced, opposed, oppressed and rejected except by those who were given to dishonesty, deceit and hypocrisy.
“It is only those who believe not in the Signs of God, who fabricate falsehood, and it is they who are the liars.” (Quran 16:105)
That is regards to falsehood in faith. As for falsehood in deed, God states in the Quran.
“...that He may try you, which of you is the best in deeds.” (Quran 67:2)
A scholar from the early period of Islam, Fudail bin Iyaad, commented on this verse, explaining:
“‘which of you Is the best in deeds’ means ‘the most sincere and correct.’ If the deed is sincere and not correct, it will not be accepted, and if it is correct and not sincere, it will not be accepted. It will not be accepted until it is both sincere and correct!”
One everyday example of where sincerity and correction of action are often subverted by falsehood is in the buying and selling of goods. Hence we find the Prophet saying:
“If they (two parties meeting to trade) are truthful and clarify (any and all deficiency in their goods), their transaction will be blessed. But if they lie and conceal (any deficiency in their goods), the blessings of their transaction will be eradicated.”
And what of falsehood in speech? Falsehood of the tongue, or what is more commonly referred to as lying, is a characteristic roundly rejected by the entire world – even if its inhabitants may fall into it from time to time. After all, if God would punish his last and greatest Prophet in case he were to lie…
“If he (Prophet Muhammad) had invented false sayings concerning Us, We would surely have grasped him firmly, and then cut off the artery of his heart, and none of you could have withheld Us from doing this.” (Quran 69:44-7)
…then how could lying be acceptable from anyone else besides His Prophet?! And he, the Prophet Muhammad, the Truthful One, said:
“A slave’s faith will not be upright until his heart is upright, and his heart will not be upright until his tongue is upright, and a man whose neighbor is not safe from his harm will not enter Paradise.”
The Prophet said: “A person lies and lies, until he is written with God as a habitual liar.” (Saheeh Al-Bukhari)
Thus, the habitual liar is despised, truly and thoroughly despised, by all – even his own kind – as no-one can trust a liar, not even other liars. And just as clarity in speech is a sign of truthfulness, so then ambiguity, innuendo, sarcasm and every other form of deception and trickery of the tongue is denounced in Islam. Even lying in jest has been condemned by the Prophet when he said:
“I guarantee a house in the middle of Paradise for the one who leaves off lying even if it be in jest.”
...and his saying:
“Woe to the person who lies to make people laugh! Woe to him, woe to him!”
The Prophet’s closest friend and immediate temporal successor, Abu Bakr as-Şiddeeq (i.e. the truthful - so named by the Prophet due to the truthfulness of his faith), further said:
“Beware of lying, for lying opposes (true) faith.”
And the daughter of Abu Bakr, Aisha, who was the beloved wife of the Prophet, mentioned that:
“There was no trait more abhorrent to the Messenger of God, may the mercy and blessings of God be upon him, than lying.”
Suffice as a deterrence from lying is its being listed as a trait of that most wretched of conditions: hypocrisy. The Prophet Muhammad said:
“The signs of the hypocrite are three: when he speaks he lies; when he makes an oath he breaks it; and when he is entrusted with something he betrays that trust.”
Not only do we learn about the abhorrence of directly lying itself, but Islam also mercifully educates us as to the dangers of all that which indirectly leads to lying.
Again from Aishah we learn that the Prophet would invoke his Lord, praying: “O God! I seek refuge with you from all sins, and from being in debt.” When asked: “O Messenger of God! You frequently seek refuge with God from being in debt!” The Prophet of God replied: “If a person is in debt, he tells lies when he speaks, and breaks his promises when he promises.”
In the same vain, the Prophet explicitly ordered his followers:
“Leave that which causes you doubt for that which does not cause you doubt. For in truthfulness lies tranquility, and in lying lies doubt.”
Striving for truthfulness then, in spirit, word and deed, is a matter which requires the utmost steadfastness from the believer, as well as the utmost vigilance against the dangers of falsehood, insincerity, deceit and hypocrisy:
“That God will reward the People of Truth for their truth, and punish the hypocrites if that be His Will, or turn to them in Mercy; for God is Oft-Forgiving, Most Merciful.” (Quran 33:24)
 The Companion, Anas b. Malik, reported that the Prophet even mentioned how: ‘A Prophet does not (even) wink!’ (Abu Dawud, Nisaa'ee, Hakim, Ahmad)
 Reported by Hakim b. Hizam, in Saheeh Al-Bukhari and Saheeh Muslim.
 Reported by the Companion, Anas b. Malik in As-Saheehah.
 Reported by Abu Umamah, in At-Tirmidhi.
 Reported by Mu‘awiyah b. Jaydah al-Qushayri in Abu Dawud.
 Bayhaqi, Shu‘ab al-Iman.
 Reported by the Companion, Abu Hurayrah, in Saheeh Al-Bukhari and Saheeh Muslim.
 Saheeh Al-Bukhari.
 Reported by Al-Hasan b. Ali, in At-Tirmidhi.