Al-Hasan reported: Abu Darda, may Allah be pleased with him, said, “Be a scholar, a student, a devotee, or a follower; do not be the fifth type lest you be ruined.” It was said to Al-Hasan, “Who is the fifth type?” Al-Hasan said, “A heretic.”

Source: al-Madkhal ila? al-Sunan al-Kubra? 277