It is reported that Abū Al-Dardā` – Allāh 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.
Ḥumayd (one of the reporters) asked Al-Ḥasan (Al-Baṣri, who reported this from Abū Al-Dardā`), “And who is the fifth?” He replied, “A heretic (mubtadi’, religious innovator}.”

Ibn ʿAbd Al-Barr, Jāmi’ Bayān Al-‘Ilm 1:142.