It is narrated from Hazrat Abu Malik Ash'ari (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: I command you with five things: listening, obeying, staying attached to the community of Muslims, migration, and striving in the way of Allah. Then whoever separates from the community of Muslims even by a handspan, he removes the rope of Islam from his neck. And whoever calls out with the call of the days of ignorance, he is the fuel of Hell. A man asked, "O Messenger of Allah! Even if he prays and fasts?" The Noble Prophet (peace and blessings be upon him) replied, "Yes! Even if he fasts and prays. However, O servants of Allah! Call people by the names that Allah has given them, that is, Muslim and believer."
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 22910