Sahih Adab al-Mufrad

Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that he (peace be upon him) said: "By the One in Whose hand is my soul, you will not enter Paradise until you become Muslims, and you will not become Muslims until you love one another. Spread the greeting of peace among yourselves, for this will cause you to love one another, and beware of mutual hatred, for indeed it is the shaver. I do not say that it shaves hair, but rather it shaves the religion."