It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "You will not enter Paradise until you believe, and you will not (truly) believe until you love one another. Shall I not tell you of something which, if you do it, you will love one another? Spread the greeting of salam among yourselves."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10358