Sahih Muslim Book 1, Hadith Number 96.

Chapter : Concerning the fact that no one will enter Paradise except believers, that love for believers is (a characteristic (feature of) Faith and giving currency to (the practice of paying salutation by, saying) As-Salimu ‘Alaikum (peace be upon you) is the means to achieve it (Faith).

Abu Huraira reported: 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.