It is reported that Abû Al-Dardâ` – Allâh be pleased with him – said:

Be a scholar or a learner, or a person who loves [the scholars] or a follower [of the scholars], but do not be the fifth.
Humayd (one of the reporters) asked Al-Hasan (Al-Basri, who reported this from Abû Al-Dardâ`), “And who is the fifth?” He replied, “A heretic (mubtadi’, religious innovator}.”

Ibn ‘Abd Al-Barr, Jâmi’ Bayân Al-‘Ilm 1:142.