Ibn Al-Qayyim (rahimahullaah) said:
Beware of falsehood because it spoils your natural ability to gain information, and it spoils your information and your ability to teach people. The liar describes that which does not exist, and that which exists he says it does not exist just as he describes truth as falsehood and falsehood as truth, and good as evil and evil as good. It spoils his conception and his knowledge, which then becomes a punishment. Then he describes what is not true to the person who trusts him. The heart of a liar is far removed from the truth, and is inclined to that which does not exist and he prefers falsehood. When his ability to perceive has been spoiled as well as his knowledge, which is the basis of every voluntary deed, these deeds would also be spoiled and the principle of falsehood would come into force.
Therefore, it would proceed from him just like falsehood proceeds from the tongue, and so, he would not benefit from either his tongue or his deeds. This is the reason why falsehood is the foundation of evil, just as the Prophet (sallallaahu alayhi wa sallam) said:
“Truthfulness leads to righteousness, and righteousness leads to Paradise. And a man keeps on telling the truth until he becomes a truthful person. Falsehood leads to Al-Fajur (i.e wickedness, evil-doing), and Al-Fajur (wickedness) leads to the (Hell) fire, and a man may keep on telling lies until he is written before Allah, as a liar.”
[Recorded by Al-Bukhari, Kitab Al-Adab, section of the word of Allah, the Exalted, no.6094., and Muslim 2606]
When falsehood proceeds from the heart to the tongue, it spoils it, then it goes to the body, and spoils the good deeds just like it spoiled the words of the tongue. Then falsehood prevails in his words, deeds and state. Therefore, corruption would become firmly embedded in him and its disease would extend to a state of destruction if Allah, the Almighty, does not cure him with the remedy of truthfulness, which cuts off evil at its source. That is why the origin of all the good deeds of the hearts is truthfulness, and their opposites such as ostentation, conceit, arrogance, pride, haughtiness, ingratitude, merrymaking, weakness, laziness, cowardice, disgrace and so on have falsehood as the origin. Therefore the origin of every good deed done secretly or openly, is truthfulness. And the origin of every evil deed, done secretly or openly, is falsehood.
Allah, the Almighty in return, rewards the liar by frustrating his interests, and rewards the truthful by helping him achieve his interests concerning this world and the Hereafter. Therefore, nothing can make us benefit more in this world and the hereafter like truthfulness, and in the same way, nothing can make us earn more evil in this world and the Hereafter like falsehood.
“O you who believe! Be afraid of Allah, and be with those who are true (in words and deeds).”
[Surat At-Tawbah 9:119]
Allah, the Almighty said again,
“Allah will say: “This is a Day on which the truthful will profit from their truth…”
[Surat Al-Ma’idah 5:119]
He, the Exalted said again,
“…And when the matter (preparation for Jihad) is resolved on, then if they had been true to Allah, it would have been better for them…”
[Surat Muhammad 47:21]
The Exalted also said:
“And those who made excuses from the bedouins came (to you, O Prophet) asking your permission to exempt them (from the battle), and those who had lied to Allah and His Messenger sat at home (without asking the permission for it); a painful torment will seize those of them who disbelieve. ”
[Surat At-Tawbah 9:90]
From: Al-Fawaa’id – Pages 228-231