Musnad Ahmad 30:2581

Waki' narrated to us: Al-A'mash narrated to us, from Abu Salih, from Abu Hurairah, 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. Shall I not direct you to something which, if you do it, you will love one another? Spread the greeting of peace (salam) among yourselves."