On the authority of 'Abdullāh ibn 'Umar who said, the Messenger عليه السلام said:
*"The salām [i.e. greeting of Islām] comes before asking a question, whoever begins with some (other) speech before the salām then do not respond to him."*
As-Silsilah As-Sahīhah, Hadith 816 (2/459)
Related to the above, the Lajnah Ad-Dā`imah (Permanent Committee of Scholarly Research & Verdicts) were asked about responding to one who does not begin with the salām. They stated that it is upon you to respond to them then advise them that the Sunnah is to begin with the salām.
Begin with salam, Islamic etiquette of delivering speech, start speech with islamic greeting