It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever causes division in the community of the Muslims while the Muslims are in a state of firmly united Islam has thrown off the collar of Islam from his neck."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10879