An-Najwa are only from the Shaytan, in order that he may cause grief to the believers. But he cannot harm them in the least, except as Allah permits. And in Allah let the believers put their trust.
Allah states that secret talks, where the believers feel anxious, are
مِنَ الشَّيْطَانِ لِيَحْزُنَ الَّذِينَ امَنُوا
(only from Shaytan, in order that he may cause grief to the believers).
meaning, that those who hold such counsels do so because of the lures of the devil,
لِيَحْزُنَ الَّذِينَ امَنُوا
(in order that he may cause grief to the believers).
The devil seeks to bother the believers, even though his plots will not harm the believers, except if Allah wills it.
Those who are the subject of evil Najwa, should seek refuge in Allah and put his trust in Him, for none of it will harm them, Allah willing. The Sunnah also forbids the Najwa so that no Muslim is bothered by it.
Imam Ahmad recorded that Abdullah bin Mas`ud said that the Messenger of Allah said,
إِذَا كُنْتُمْ ثَلَثَةً فَلَ يَتَنَاجَى اثْنَانِ دُونَ صَاحِبِهِمَا فَإِنَّ ذلِكَ يُحْزِنُه
If you were three, then two of you should not hold a secret counsel in the presence of the third person, because that would cause him to be worried.
This Hadith is collected in the Two Sahihs using a chain of narration that contained Al-A`mash.
Abdur-Razzaq narrated that Abdullah bin Umar said that Allah's Messenger said,
إِذَا كُنْتُمْ ثَلَثَةً فَلَ يَتَنَاجَى اثْنَانِ دُونَ الثَّالِثِ إِلاَّ بِإِذْنِهِ فَإِنَّ ذلِكَ يُحْزِنُه
If you were three, then two of you should not hold a secret counsel in the presence of the third person, except with his permission, because that would cause him to be worried.
Muslim collected this Hadith