Musnad Ahmad 30:3519

Aswad ibn Amir narrated to us, Abu Bakr narrated to us, from Asim, from Abu Salih, from Abu Hurairah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "By Allah, you will not enter Paradise until you believe, and you will not believe until you love one another." He said: "If you wish, I will guide you to that which, if you do it, you will love one another: spread the greeting of peace among yourselves."