Musnad Ahmad 7:8

Yazid ibn Harun narrated to us, Hisham informed us, from Yahya ibn Abi Kathir, from Ya'ish ibn al-Walid ibn Hisham, and Abu Mu'awiyah Shayban, from Yahya ibn Abi Kathir, from Ya'ish ibn al-Walid ibn Hisham, from al-Zubayr ibn al-Awwam, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There has crept into you the disease of the nations before you: envy and hatred. And hatred is the 'shaver' (al-haliqah); it shaves the religion, not the hair. By the One in whose hand is the soul of Muhammad, you will not believe until you love one another. Shall I not inform you of something that, if you do it, you will love one another? Spread the greeting of peace (salam) among yourselves."