‘Umar b. Al-Khattāb – Allāh be pleased with him – said:

Verily, the followers of opinion are the enemies of the Sunan (the teachings of Allāh’s Messenger as passed down in hadith): they were unable to preserve them and their meanings escaped them, and when asked [questions] they were too embarrassed to say ‘We don’t know,’ so they opposed the Sunan with their opinions.

Ibn Abī Zamanīn, Uṣūl Al-Sunnah article 8; Al-Lālakā`ī, Sharḥ Uṣūl I’tiqād Ahl Al-Sunnah article 201 et al.