Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) reports this saying of the Prophet (peace and blessings be upon him): "Satan is a wolf to man, just as the wolf is a danger to the sheep; it seizes the sheep that strays far off and keeps apart. So beware of going into the mountain passes (or of splitting into different factions)! Hold fast to the congregation, to the general body of people, and to the mosque."
Ahmad recorded it; the narrator al-Ala' bin Ziyad did not hear from Sayyiduna Mu'adh (may Allah be pleased with him).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2033