Musnad Ahmad 937:5

Abu al-Yaman narrated to us, Ibn 'Ayyash narrated to us, from al-Bakhtari ibn 'Ubayd ibn Salman, from his father, from Abu Dharr, from the Prophet, may God bless him and grant him peace, that he said: "Two are better than one, and three are better than two, and four are better than three. So you must adhere to the group (al-jama'ah), for God will not gather my nation except upon guidance."