Surah At-Taubah, Ayat 107

Ayat 107. And as for those who put up a mosque by way of harming and disbelief, and to disunite the believers, and as an outpost for those who warred against Allah and His Messenger (Muhammad  [SAWW](PBUH) aforetime, they will indeed swear that their intention is nothing but good. Allah bears witness that they are certainly liars. 

Hazrat Abdullah bin Mas’ud (May Allah be pleased with him) reported: The Prophet  [SAWW](PBUH) 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". [Al-Bukhari Hadith # 6094 and Muslim Hadith # 6306].  

Lesson : as mentioned above in Surah At-Taubah Ayat 107 "Allah bears witness that they are certainly liars". 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 Him 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.