Musnad Ahmad 30:3693

'Abd al-Samad and Hasan ibn Musa narrated to us, they said: Hammad narrated to us, from Suhayl, 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."