Musnad Ahmad 28:160

Hassan narrated to us, Ibn Lahi'ah narrated to us, Darraj narrated to us, from Isa ibn Hilal al-Sadafi, from Abdullah ibn Amr ibn al-As, from the Messenger of Allah, peace and blessings of Allah be upon him, who said: "Indeed, the souls of the believers meet at a distance of a day's journey, though none of them has ever seen the other."