Better Say: 'I don't know' ..
Imaam Maalik Ibn Anas (d.179H) – rahimahullaah – said:
“The shield of the scholar is saying, ‘I do not know.’ So if he forgets it, then his enemies strike him!” [Hilyatul-Awliyaa 6/323]
Al-Haytham Ibn Jameel said:
“I heard Maalik being asked about forty-eight questions, he replied to thirty-two of them saying, ‘I do not know.’”
’Abdur-Rahmaan Ibn Mahdee (d.198H) – rahimahullaah – said:
“[I]We were with Maalik Ibn Anas when a man came saying, ‘O Abaa ’Abdullaah, I have traveled a distance of six months. The people of my land sent me to ask you about a matter.’ So he said, ‘Ask!’ So the man asked him some things, so he replied, ‘I do not know.’ – and the man thought he had come to someone who knew everything, so he said, ‘What shall I tell the people of my land when I return to them?’ He said, ‘Say to them, Maalik Ibn Anas does not know.’”
Several of the Salaf, including Abu Al-Dardaa’ :anhu:, were quoted to have said: It is half of knowledge to say “I do not know”. (Siyar A’lam Al-Nubalaa’ by Imam Thahabi 8/77)