Hadith: Adhere to truthfulness, for truthfulness leads to righteousness, and righteousness leads to Paradise

عَنْ عَبْدِ اللهِ بنِ مَسعُودٍ رضي الله عنه قَالَ: قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ:
«عَلَيْكُمْ بِالصِّدْقِ، فَإِنَّ الصِّدْقَ يَهْدِي إِلَى الْبِرِّ، وَإِنَّ الْبِرَّ يَهْدِي إِلَى الْجَنَّةِ، وَمَا يَزَالُ الرَّجُلُ يَصْدُقُ وَيَتَحَرَّى الصِّدْقَ حَتَّى يُكْتَبَ عِنْدَ اللهِ صِدِّيقًا، وَإِيَّاكُمْ وَالْكَذِبَ، فَإِنَّ الْكَذِبَ يَهْدِي إِلَى الْفُجُورِ، وَإِنَّ الْفُجُورَ يَهْدِي إِلَى النَّارِ، وَمَا يَزَالُ الرَّجُلُ يَكْذِبُ وَيَتَحَرَّى الْكَذِبَ حَتَّى يُكْتَبَ عِنْدَ اللهِ كَذَّابًا».
[صحيح] - [متفق عليه] - [صحيح مسلم: 2607]

‘Abdullah ibn Mas‘ūd (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) said:
"Adhere to truthfulness, for truthfulness leads to righteousness, and righteousness leads to Paradise. A man will keep telling the truth and strive to tell the truth until he is recorded with Allah as the most truthful. Beware of lying, for lying leads to wickedness and wickedness leads to Hellfire. A man will keep telling lies and strive to tell lies until he is recorded with Allah as a persistent liar."
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Muslim - 2607]

Explanation

The Prophet (may Allah's peace and blessings be upon him) enjoined truthfulness and informed that adherence to it leads to constant good deeds. Persistence in doing good deeds leads the doer to Paradise, and he continues to adopt truthfulness in secret and public and deserves to be labeled as most truthful - this is extreme truthfulness. Then, he (may Allah's peace and blessings be upon him) warned of lying and false speech, for it prompts a person to deviate from uprightness and to engage in evil, corruption, and sins; then, it leads him to Hellfire. He continues to tell more and more lies until he is recorded with Allah among the liars.