How serious is lying?
Truthfulness leads to righteousness and Paradise; lying leads to wickedness and the Fire, until a person is written with Allah as a liar. The Prophet ﷺ repeatedly warned against false statements and false witness among the greatest sins. Be with the truthful.
Evidence
يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُواْ ٱتَّقُواْ ٱللَّهَ وَكُونُواْ مَعَ ٱلصَّـٰدِقِينَ
O you who have believed, fear Allāh and be with those who are true.
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
Narrated `Abdullah: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."
أَلاَ أُنَبِّئُكُمْ بِأَكْبَرِ الْكَبَائِرِ … الإِشْرَاكُ بِاللَّهِ، وَعُقُوقُ الْوَالِدَيْنِ … أَلاَ وَقَوْلُ الزُّورِ وَشَهَادَةُ الزُّورِ
Narrated Abu Bakra:Allah's Messenger (ﷺ) said thrice, "Shall I not inform you of the biggest of the great sins?" We said, "Yes, O Allah's Messenger (ﷺ)" He said, "To join partners in worship with Allah: to be undutiful to one's parents." The Prophet (ﷺ) sat up after he had been reclining and added, "And I warn you against giving forged statement and a false witness; I warn you against giving a forged statement and a false witness." The Prophet kept on saying that warning till we thought that he would not stop.
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