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 ilā al-Sunan al-Kubrá 277
عَنِ الْحَسَنِ أَنَّ أَبَا الدَّرْدَاءِ رضي الله عنه قَالَ كُنْ عَالِمًا أَوْ مُتَعَلِّمًا أَوْ مُحِبًّا أَوْ مُتَّبَعًا وَلا تَكُنِ الْخَامِسَ فَتَهْلِكَ قيل لِلْحَسَنِ مَنِ الْخَامِسُ قَالَ الْمُبْتَدِعُ
277 المدخل إلى السنن الكبرى للبيهقي