Praise be to Allah.
It is not permissible to greet a Muslim by saying “peace be upon the one who follows true guidance”, because the Messenger only used this greeting when he wrote to non-Muslims. When you greet your Muslim brother you should say to him, al-salaamu ‘alaykum (peace be upon him). If you say, Al-salaam ‘ala man attaba’a al-huda (peace be upon the one who follows true guidance), what this implies is that this brother of yours is not one of those who follow true guidance. If there is a group of Muslims and Christians, then one should greet them in the usual manner by saying al-salaamu ‘alaykum, intending that to be addressed to the Muslims. End quote.
Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him)
Fataawa al-‘Aqeedah, p. 237
See also the answer to question number 105399 .