‘Umar b. Al-Khattâb – Allâh be pleased with him – said:

Verily, the followers of opinion are the enemies of the Sunan (the teachings of Allâh’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 Abî Zamanîn, Usûl Al-Sunnah article 8; Al-Lâlakâ`î, Sharh Usûl I’tiqâd Ahl Al-Sunnah article 201 et al.