Question: Many people lift their hand up to the forehead while greeting a fellow Muslim, is this correct to do so at the time of greeting? 

Answer: Raising hand during greeting a fellow Muslim is not the Sunnah method, rather this kind of act is a trend of other religion which is forbidden by Rasulullah s.a.w. Every religion has their own trend and way of lifestyle hence, it is stated in the hadith:

The Messenger of Allah s.a.w said: “He is not one of us who resembles other than us, no who resembles the Jews nor the Christians. For indeed greeting of the Jews is pointing the finger, and the greeting of the Christians is waving with the hand.”   

References: [1] Tirmidhi: 2695 [2] Ahsanul Fatwa: vol-8, p-143