Ibn Sina and Ibn Maskooya
From: Anecdotes of the Pious Ones, # 20, v.1

Abu 'Ali ibn-e Sina wasn't twenty years of age while he had already learned the knowledge of his time. He was the most learned person of his time in religeous and natural sciences, and mathematics.

One day he went to Abu 'Ali ibn Maskooya, the famous scholar of that time. Arrogantly, he threw a walnut to him and asked him to determine its area.

Ibn Maskooya gave him a few booklets on the importance of ettiquetes (The book of Taharat al A'raq) and told him: "You modify your character so that I may determine the area of the walnut. You are more in need of rectifying your character than I am of determining the area of this walnut."

Ibn Sina became ashamed of his character and used this admonition as the guide of his life.[1]

[1] The History of Sciences in Islam. p. 211