As-salamu alaykum (Arabic: ٱلسَّلَامُ عَلَيْكُمْ, as-salāmu ʿalaykum), also written salamun alaykum and typically rendered in English as salam alaykum, is a greeting in Arabic that means 'Peace be upon you' (wikipedia).

The use of the phrase has its roots in the Quran and the Hadith, holy books in Islam. Muslims are instructed to return greetings courteously in Surah an-Nisa: 86 of the Quran ( Dictionary )

In the Hadith book Sahih al-Bukhari, Volume 8, Book 74, Number 246, Adam, newly created, greets the angels of Allah with assalamu alaikum.

Today, The salām (سَلَام, meaning 'peace') has become a salutation for Muslims & Arabs worldwide when greeting each other