The Prophet ﷺ said, "Truthfulness leads to righteousness, and righteousness leads to Paradise. And a man keeps on telling the truth until he becomes a truthful person. Falsehood leads to Al-Fajur (i.e. wickedness, evil-doing), and Al-Fajur (wickedness) leads to the (Hell) Fire, and a man may keep on telling lies till he is written before Allah, a liar."  

عَنِ النَّبِيِّ ﷺ قَالَ: «إِنَّ الصِّدْقَ يَهْدِي إِلَى الْبِرِّ وَإِنَّ الْبِرَّ يَهْدِي إِلَى الْجَنَّةِ، وَإِنَّ الرَّجُلَ لَيَصْدُقُ حَتَّى يَكُونَ صِدِّيقًا۔ وَإِنَّ الْكَذِبَ يَهْدِي إِلَى الْفُجُورِ وَإِنَّ الْفُجُورَ يَهْدِي إِلَى النَّارِ، وَإِنَّ الرَّجُلَ لَيَكْذِبُ حَتَّى يُكْتَبَ عِنْدَ اللَّهِ كَذَّابًا۔»  

I said to al-Ḥasan b. ʿAlī: What did you memorize from the Messenger of Allah ﷺ? He said: I memorized from him: "Leave what makes you doubt for what does not make you doubt. The truth brings tranquility while falsehood sows doubt."  

قُلْتُ لِلْحَسَنِ بْنِ عَلِيٍّ: مَا حَفِظْتَ مِنْ رَسُولِ اللَّهِ ﷺ؟ قَالَ: حَفِظْتُ مِنْ رَسُولِ اللَّهِ ﷺ: «دَعْ مَا يَرِيبُكَ إِلَى مَا لاَ يَرِيبُكَ، فَإِنَّ الصِّدْقَ طُمَأْنِينَةٌ وَإِنَّ الْكَذِبَ رِيبَةٌ۔»