It is narrated from Uzrah Tamimi that Sayyiduna Ali (may Allah be pleased with him) said three times: "It is the coolness of the heart." The people asked: "What is that, O Commander of the Faithful?" He replied: "That a man is asked about something he does not know, and he says: «الله أعلم»"
Hadith Takhrijیہ اثر ضعیف ہے۔ دیکھئے: [جامع بيان العلم 1569] و [المدخل للبيهقي 794] اس کی سند میں محمد بن حمید ضعیف ہے۔
Brief Explanation
(Commentary on Ahadith 179 to 184) In all these traditions, it is mentioned that by saying «اللّٰه أعلم», one attains peace and tranquility of the heart, and a person is saved from unnecessary affectation.