Allah Ta’ala says:
“Refrain from false statements.”
Rasulullah (saws) said: “Make truth incumbent on you and beware of falsehood.”
Falsehood is to speak contrary to facts. For a person to be regarded as a liar it will suffice that he narrates every rumour without investigating whether it is a fact or not. Rasulullah (saws) ordered abstention from lies because falsehood and immorality are complementary partners and both will be in Jahannum. Nabi (saws) said that false evidence is equivalent to shirk.
In a vision Rasulullah (saws) was shown a man whose cheeks were being repeatedly slit from ear to throat. The cheeks would heal immediately upon having been slit and the process of slitting would be repeated. Upon enquiring, Rasulullah (saws) was informed by Jibra’eel (alayhis salaam) that the one who was being thus punished was a liar and this punishment will continue in the grave until Qiyaamah.
In another hadith it is narrated that a woman called her child. To entice the child in coming to her she indicated to the child that if he came, she would give him something. Rasulullah (saws) asked her that if the child came, what would she give him? She replied that she would give dates. Rasulullah (saws) then commented that if she had no intention of giving the child anything, her statement merely being to lure the child to her, then such statement is also a lie.
When speaking, be careful. Do not speak without thinking. Ponder before you speak and be firm in confronting and curbing the urge to speak what is false. If falsehood was spoken then compensate this error by means of istighfaar. Should any word contrary to the Shariah slip from your mouth, resort to Taubah in abundance.