Umar bin Khattab (radiyAllaahu 'anhu) said:
"Beware of the people of opinion because
they are the enemies of sunan. The hadith
exhausted them in order for them to
memorize them, so they said according to
opinions, so they went astray and led others
astray."
[Ilam Muaqeen of Ibn Qayyim (1/55) and
authenticated by Ibn Hajar Asqalani in Fath al
Bari (13/289)]