Al-Matalib al-'Aliyah 2147 — Urging obedience, and that the religion

Ishaq (may Allah have mercy on him) said: Baqiyyah bin al-Walid informed us; Thabit bin al-Ajlan narrated to us; he said: A man who had heard the like of it narrated to me, and he was one of the eminent Companions of the Prophet (peace and blessings be upon him), that the Mother of the Believers, Sayyidah Umm Salamah (may Allah be pleased with her), wrote to the people of Iraq: "Allah Almighty is free of, and He has declared His Messenger (peace and blessings be upon him) free of, whoever forms factions and separates from the community (jama'ah). So never form factions and never separate from the community. Wa al-salamu alaykum wa rahmatullah (Peace be upon you and the mercy of Allah)."
This narration is transmitted through the chain of Imam Ishaq bin Rahwayh (may Allah have mercy on him), in which this letter of Sayyidah Umm Salamah (may Allah be pleased with her) is related through a Companion of the Messenger of Allah (peace and blessings be upon him).