Abu Hurayrah (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) said: "By the One in Whose Hand my soul is! You will not enter Paradise until you believe and you will not believe until you love one another other. Shall I tell you of something that, if you do it, you will love one another? Spread the greeting of peace among yourselves." [Muslim].