In The Name of Allah, The Most Merciful, The Bestower of Mercy.

Allah, The Most High, said:

يَوْمَ يَبْعَثُهُمُ ٱللَّهُ جَمِيعًا فَيُنَبِّئُهُم بِمَا عَمِلُوٓا۟ أَحْصَىٰهُ ٱللَّهُ وَنَسُوهُ وَٱللَّهُ عَلَىٰ كُلِّ شَىْءٍ شَهِيدٌ
أَلَمْ تَرَ أَنَّ ٱللَّهَ يَعْلَمُ مَا فِى ٱلسَّمَٰوَٰتِ وَمَا فِى ٱلْأَرْضِ مَا يَكُونُ مِن نَّجْوَىٰ ثَلَٰثَةٍ إِلَّا هُوَ رَابِعُهُمْ وَلَا خَمْسَةٍ إِلَّا هُوَ سَادِسُهُمْ وَلَآ أَدْنَىٰ مِن ذَٰلِكَ وَلَآ أَكْثَرَ إِلَّا هُوَ مَعَهُمْ أَيْنَ مَا كَانُوا۟ ثُمَّ يُنَبِّئُهُم بِمَا عَمِلُوا۟ يَوْمَ ٱلْقِيَٰمَةِ إِنَّ ٱللَّهَ بِكُلِّ شَىْءٍ عَلِيمٌ

On the Day that Allāh will resurrect them all (from their graves) and inform them of what they did (of evil) while they (will have) forgotten it. Allāh has enumerated it (all), and Allāh is a witness over everything. Have you not considered that Allāh knows what is in the heavens and what is in the earth (and that nothing remains hidden from Him)? There is no secret counsel of three except that He is their fourth, nor of five except that is their sixth, and nor less than that or greater except that He is with them (through His all-encompassing hearing, seeing and knowledge) wherever they may be. Then He will inform them (the hypocrites and their allies) of what they did on the Day of Resurrection. Indeed, Allāh is all-knowing of everything.

أَلَمْ تَرَ إِلَى الَّذِينَ نُهُوا عَنِ النَّجْوَىٰ ثُمَّ يَعُودُونَ لِمَا نُهُوا عَنْهُ وَيَتَنَاجَوْنَ بِالْإِثْمِ وَالْعُدْوَانِ وَمَعْصِيَتِ الرَّسُولِ وَإِذَا جَاءُوكَ حَيَّوْكَ بِمَا لَمْ يُحَيِّكَ بِهِ اللَّهُ وَيَقُولُونَ فِي أَنفُسِهِمْ لَوْلَا يُعَذِّبُنَا اللَّهُ بِمَا نَقُولُ ۚ حَسْبُهُمْ جَهَنَّمُ يَصْلَوْنَهَا ۖ فَبِئْسَ الْمَصِيرُ
يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا تَنَاجَيْتُمْ فَلَا تَتَنَاجَوْا بِالْإِثْمِ وَالْعُدْوَانِ وَمَعْصِيَتِ الرَّسُولِ وَتَنَاجَوْا بِالْبِرِّ وَالتَّقْوَىٰ ۖ وَاتَّقُوا اللَّهَ الَّذِي إِلَيْهِ تُحْشَرُونَ

Have you not considered those (hypocrites and their allies) who were prohibited from secret counsels, then they revert to that which they were prohibited from, and converse among themselves with sin and (plots of) aggression (against the believers) and disobedience to the Messenger? And when they (the hypocrites) come to you, they greet you (with pre-Islāmic greetings) (or: they, the Jews, greet you with words intended as a curse) by that which Allāh does not greet you (which is “salām”, the greeting of peace) and they say among themselves: “(If Muḥammad is indeed a prophet) why does Allāh not punish us for what we say?” Sufficient for them is (the) Hell that they will burn in, and what an evil destination (it is). O you who believe (or: O you who hypocrites who claim to believe) (or: O you who believe in Moses), when you hold secret counsel then do not converse with (speech that involves) sin, (and which is about) aggression and disobedience to the Messenger, but converse with righteouness and piety. And fear Allāh to Whom you will be gathered. [1]

Imam As-Sadi, may Allah have mercy upon him, said:

التناجي بين اثنين فأكثر، وقد تكون في الخير، وتكون في الشر.
النجوى هي: التناجي بين اثنين فأكثر، وقد تكون في الخير، وتكون في الشر.
فأمر الله تعالى المؤمنين أن يتناجوا بالبر، وهو اسم جامع لكل خير وطاعة، وقيام بحق لله ولعباده والتقوى، وهي [هنا]: اسم جامع لترك جميع المحارم والمآثم، فالمؤمن يمتثل هذا الأمر الإلهي، فلا تجده مناجيا ومتحدثا إلا بما يقربه من الله، ويباعده من سخطه، والفاجر يتهاون بأمر الله، ويناجي بالإثم والعدوان ومعصية الرسول، كالمنافقين الذين هذا دأبهم وحالهم مع الرسول صلى الله عليه وسلم.

An-Najwaa (secret counsel) is between two (people) or more. It may be regarding (something) good and maybe evil. Therefore, Allah, The Most High, commanded the believers to converse privately (in pusruit) of Bir, which (i.e. Bir) is comprehensive term that includes every good and (act of) obedience (to Allah and His Messenger), fulfilment of Allah’s rights and the (rights of) His servants. And Taqwa (in this context) is a comprehensive term regarding the abandonment of all prohibitions and sins. A believer fulfils this divine command, and you will not find him in private counsel and conversation except upon that which draws him closer to Allah and distance him from His wrath. (As for the) Fajir (evil sinner), he is apathetic towards Allah’s command and engages in private counsel for sin, transgression and disobedience to the Messenger, like the hypocrites whose normal practice and situation is as such towards the Messenger, peace and blessings of Allah be upon him. [2]

[1] https://www.thenoblequran.com/q/#/verse/58/6 https://www.thenoblequran.com/q/#/verse/58/7
https://www.thenoblequran.com/q/#/verse/58/8 https://www.thenoblequran.com/q/#/verse/58/9

[2] Paraphrased from Tasfir As-Sadi