It is narrated from Sayyiduna Salman al-Farisi (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Souls are like assembled troops: those that recognise one another become intimate, and those that do not recognise one another differ."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6144