Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: It is only a matter of two things: guidance (conduct) and speech. The best speech is the speech of Allah, and the best guidance is the guidance of Muhammad (peace and blessings be upon him). Beware! Keep away from newly invented matters and innovations, for the worst of things are newly invented matters, and every newly invented thing is misguidance. Beware! Let not the term become long for you so that your hearts become hardened by long hopes; what is coming is near. Beware! The far is only that which is not coming. Beware! The wretched one is he who was written as wretched while still in his mother's womb, and the fortunate one is he who takes admonition from the state of others. The worst narration is a false narration. Beware! Lying is not right, neither in earnest nor in jest. Beware! Lying leads to wickedness, and wickedness leads to Hell; and truthfulness leads to righteousness, and righteousness leads to Paradise. It is said of the truthful person: "He spoke the truth and acted righteously," and it is said of the liar: "He lied and acted wickedly."
And I heard the Messenger of Allah (peace and blessings be upon him) say: "A servant keeps lying until he is written with Allah as a great liar, and he keeps speaking the truth until he is written as a siddiq (truthful one)" - then he said - "Beware of 'adh (slanderous lying). Do you know what 'adh is? It is tale-bearing and carrying words from one person to another."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8486