Musnad Ahmad 30:817

Yazid narrated to us, Hammad ibn Salamah informed us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Souls are recruited soldiers; those of them that recognize one another become compatible, and those of them that do not recognize one another become at variance."