O you who have believed, when you converse privately, do not converse about sin and aggression and disobedience to the Messenger but converse about righteousness and piety. And fear Allāh, to whom you will be gathered.
(Have you not seen those who were forbidden to hold secret counsels, and afterwards returned to that which they had been forbidden,) He said, "The Jews." Similar was said by Muqatil bin Hayyan, who added, "The Prophet had a peace treaty with the Jews. When one of the Prophet's Companions would pass by a gathering of Jews, they would speak among themselves in secret, prompting the believer to think that they were plotting to kill or harm him. When the believer saw this, he feared for his safety and changed the route he was taking. The Prophet advised them to abandon their evil secret talks, but they did not listen and kept on holding the Najwa. Allah the Exalted sent down this Ayah in their case,
يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا تَنَاجَيْتُمْ فَلَا تَتَنَاجَوْا بِالْإِثْمِ وَالْعُدْوَانِ وَمَعْصِيَتِ الرَّسُولِ وَتَنَاجَوْا بِالْبِرِّ وَالتَّقْوَىٰ (0 those who believe, when you hold secret counsel, do not whisper for sinful act and wrongdoing and disobedience of the Messenger, and do whisper for righteousness and taqwa [ God-fearing ]... 58:9) In this and the preceding verses secret societies of the unbelievers have been condemned, but the condemnation is not unreserved or unqualified. The current verse guides the Muslims to be conscious of the fact that Allah is fully aware of their conversation and all of their conditions. Therefore, all their secret conferences must be held for good purposes. It is the object or motive of a particular conference that determines its nature. Thus in the present verse, as indicated earlier, the believers have been allowed to hold secret conferences only to promote good and righteous causes.
Return Good for Evil
It was narrated earlier that one of the evil practices of some of the Jews of Madinah was that when they came to the Messenger of Allah ﷺ ، they greeted him by invoking curses upon him, thus: اَلسَّام عَلَیکُم As-Samu` alaykum' [ death to you!] instead of saluting him with the usual اَلسَّلَام عَلَیکُم 'As-Salamu ` alaykum' [ peace be upon you ]. The word 'As-Sam' means 'death' instead of 'As-Salam' which means 'peace'. The latter word was distorted and pronounced with a little twist of the tongue, which sounded like the former word, but the difference between the two words was so subtle that the Companions ؓ could hardly detect the difference. One day, however, Sayyidah Siddiqah 'A'ishah ؓ detected that the Jews were invoking curses upon the Messenger of Allah ﷺ by saying 'As-Samu ` alaykum' [ death to you!], so she replied: اَلسَّام عَلَیکُم وَ لَعَنَکُمُ اللہُ وَ غَضِبَ عَلَیکُم As-Samu ` alaykum wa la` anakumullah wa ghadiba ` alaykum' [ death to you, and the curse of Allah be on you, and His wrath!]. The Messenger of Allah ﷺ prevented her from this and said: "0 ` A'ishah , Allah does not like rudeness and foul speech." Sayyidah Siddiqah ` A'ishah ؓ said: "0 Messenger of Allah, did you not hear what they said." He said to her: "Did you not hear my answering them back by saying وَعَلَیکُم 'wa ` alaykum' [ same to you ]?" He further said to her 'Allah accepts our supplication against them, but not theirs against us'. This event tells us that even in response to the mischievous words of the infidels, the Muslims should always adopt a soft and noble way.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 8–9
The universe with its complex system bears testimony to the fact that it is under the careful watch of a Higher Power. The evidence of this vigilance over the universe proves that man is also constantly under the eyes of his Creator. In view of this, indulging in secret activities against Truth is the act of blind people who are unable to read the attributes of God either in the expressly worded Quran directly, or in the unworded message of the universe indirectly. When certain hypocrites used to approach the Prophet Muhammad, instead of saying assalamu alaykum (peace be on you) they used to say assamu alaykum (death be upon you). This has always been the way of superficial persons. Such people derive pleasure from showing disrespect to true believers. They forget that at that very moment the whole of God’s creation will acclaim the truthful person, while they, with their narrow mentality, will have exhausted every last word of contempt and rejection.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.