Musnad Ahmad 31:828

'Ali ibn 'Ayyash narrated to us, Muhammad ibn Mutarrif narrated to us, Abu Hazim narrated to us, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the chambers of those who love one another will be seen in Paradise like the rising star in the east or the west. It will be said: 'Who are these?' It will be said: 'These are those who loved one another for the sake of Allah, the Mighty and Majestic.'"