American Islamic Information Center
What is the history of the Islamic greeting, when did it start, and who was the one who chose it?
Imams Bukhari and Muslim reported that the prophet (S.A.W.) said, "When Allah created Adam he told him to go and say Assalamu Alikum to a group of Angels and listen to their reply. It is your greeting and the greeting of your descendants. Adam went and said: Assalamu Alikum they said Assalamu Alikum Wa Rahmatulah."
The complete form for the Islamic greeting is Assalamu Alilkum Wa Rahmatulah Wa Barakatuh meaning peace, mercy, and blessings be upon from Allah (S.W.T.). This is because Abu-Dawood and Termithi reported in a good hadith that a man came to the prophet (S.A.W.) and said, "Assalamu Alikum." The Prophet responded and the man sat down. The Prophet said, "Ten rewards." Another man came and said, "Assalamu Alikum Wa Rahmatullh." The Prophet responded and the man sat down. The Prophet said, "twenty rewards."
Then another man came and said: Assalamu Alakum Wa Rahmatulah Wa Barakatuh. The prophet responded and the man sat down. The prophet said, "thirty rewards."
The Scholars have agreed that starting with salam is highly recommended. And responding is obligatory because Allah said in surat An-Nesa’, (verse 86), what can be translated as, "When a courteous greeting is offered to you, meet it with a greeting still more courteous, or at least of equal courtesy."