Musnad Ahmad 30:1963

Aswad narrated to us, Sharik narrated to us, from al-A’mash, from Abu Salih, from Abu Hurayrah, who attributed it to the Prophet (ﷺ), who said: "You will not enter Paradise until you believe, and you will not believe until you love one another. Shall I not guide you to the foundation of that, or the essence of that? Spread peace among yourselves." And perhaps Sharik said: "Shall I not guide you to something which, if you do it, you will love one another? Spread peace among yourselves."