Sahih Muslim, Chapter: Faith (Kitab Al Iman)

The Messenger of Allah (may peace and blessing be upon him) observed: You shall not enter Paradise so long as you do not affirm belief (in all those things which are the articles of faith) and you will not believe as long as you do not love one another. Should I not direct you to a thing which, if you do, will foster love amongst you: (i. e.) give currency to (the practice of paying salutation to one another by saying) as?salamu alaikum.