Ibn Mahdi (rahimahullah) narrates: A man once asked Imaam Maalik (rahimahullah) a question. On hearing the question, Imaam Maalik (rahimahullah) replied, “I am not sure.” To this, the man responded, “I travelled to you from such-and-such a distant place in order to ask you this question.” Imaam Maalik (rahimahullah) said, “When you return to your place, then inform the people that I said to you, ‘I am not sure.’” (Hilyatul Awliyaa vol. 6 pg. 353)

Ibn Wahb (rahimahullah) has mentioned, “If I wished, I could fill multiple slates with the statement of Imaam Maalik (rahimahullah), ‘I do not know.’ (on account of him giving this reply on so many occasions)” (Hilyatul Awliyaa vol. 6 pg. 353)