﴿ إِنَّمَا ٱلنَّجْوَىٰ مِنَ ٱلشَّيْطَٰنِ لِيَحْزُنَ ٱلَّذِينَ ءَامَنُوا۟ وَلَيْسَ بِضَآرِّهِمْ شَيْـًٔا إِلَّا بِإِذْنِ ٱللَّهِ ۚ وَعَلَى ٱللَّهِ فَلْيَتَوَكَّلِ ٱلْمُؤْمِنُونَ ﴾

“[All other kinds of] secret confabulations are but of Satan’s doing, so that he might cause grief to those who have attained to faith; yet he cannot harm them in the least, unless it be by God’s leave: in God, then, let the believers place their trust!”

As a rule, secret whispering is an undesirable action. However, sometimes for some good reason, secret whispering has to be resorted to. In this case, the decisive factor is intention. If it is done with good intentions, it is legitimate but if it is done with evil intentions, it is sinful.