Musnad Ahmad 30:3301

Ibn Numayr narrated to us, al-A‘mash narrated to us, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "By the One in whose hand is my soul, you will not enter Paradise until you believe, and you will not believe until you love one another. If you wish, I will guide you to a matter that, if you do it, you will love one another." They said: "Yes." He said: "Spread the greeting of peace (salam) among yourselves."