In the name of Allah, Most Gracious, Most Merciful.
Abdullah bin Mas’ud (May Allah be pleased with him) reported: The Prophet (May Allah exalt his mention and protect him from imperfection) said: “Truth leads to piety and piety leads to Jannah. A man persists in speaking the truth till he is enrolled with Allah as a truthful. Falsehood leads to vice and vice leads to the Fire (Hell), and a person persists on telling lies until he is enrolled as a liar”. (Agreed upon)
Commentary: Siddiq (veracious) and Kadhdhab (liar) both are adjectives of intensive degree. That is to say, the words stand respectively for someone whose truth has become his second nature; and in the opposite case, one is a liar who is in the habit of telling lies. As one acquires a reputation in this world for his good or bad deeds similar is his position before Allah. One who is ranked among the truthful with Allah is entitled to reward, and if one is a liar, he has to suffer retribution for it. This Hadith provides incentive for truthfulness because it is a source of every good deed and contains a warning against lying as it gives rise to all kinds of mischief.
Riyaadiss-Saliheen (The Meadows of the Righteous)
By Imam An Nawawi
Chapter 4, Page 23, No 54