Musnad Ahmad 1018:84

Ishaq ibn Sulayman al-Razi narrated to us, he said: I heard Zakariya ibn Salam narrating, from his father, from a man who said: I reached the Prophet, may Allah bless him and grant him peace, while he was saying: "O people, you must adhere to the community (al-jama'ah), and beware of division. O people, you must adhere to the community, and beware of division." He said it three times, Ishaq said it.