Is this based on the subject of the question i.e. fiqh, aqeedah, hadith? The difficulty of the question, or simply just when one does not himself believe that he knows the answer?
Answer
Allah Ta’ala says, ‘Ask the people of Dhikr (knowledge) if you do not know.’
The verse advises to refer to the Ulama upon not knowing the answer to any
particular matter.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai