Reference: Siyar A’laam an-Nubalaa – Volume 12, Page 63
Suhnoon was asked whether it is possible for a scholar to say ‘I do not know’ in response to a matter he knows about? He replied:
As for that which [he knows] is affirmed in the Book and the Sunnah, then no [he cannot say ‘I do not know’]. As for that [matter] which is open to opinion, then yes it is possible [to say ‘I do not know’]. That is because he does not know [for sure] whether he is right or wrong.
 The Prophet (sal-Allaahu ‘alayhi wa sallam) said: