Yoosuf Ibn al-Hasan رحمه الله says: I came to know that Dhunnoon Misri رحمه الله was one of those pious men of Allaah who knew the Ismul A'dham (the greatest name of Allaah سبحانه و تعالى, through which Allaah سبحانه و تعالى fulfils people's needs) and I decided to travel to Egypt to learn it from him. I remained in his company serving him for one year, and then requested, "May Allaah have mercy upon you! I have served you for one year and feel that you are obligated to recompense me for my services. I request that you teach me the Ismul A'dham. I do not think that you will find anybody worthier of learning it than me."

He remained silent and did not reply to my request for a further six months. One day, he indicated that he would teach it to me. He entered his home and brought to me a dish which was sealed with a cloth on the top. He asked whether I knew a friend of his who resided in Fustat. I replied in the affirmative and he requested that I take the dish to him. I left Giza with the sealed dish in my hands.

As I proceeded I thought to myself, "I wonder what the great Dhunnoon Misri is sending as a gift to his friend?" I could not contain my curiosity and unfastened the cover of the dish. The moment I did so a mouse jumped out and escaped. I felt humiliated and angrily said to myself, "Dhunnoon Misri dares to mock at a person like me!"

I returned, burning with rage. The moment he saw me, he realised what had and happened and said, "O foolish man, I entrusted you with a mouse and you deceived me. How can I ever trust you with the Ismul A'dham?" With these words he asked me to leave. [Thamaraatul Awraaq, p65]

As certain forms of knowledge have the potential of resulting in harm if used incorrectly, the sages are careful who they entrust them to. [Pearls from the Path]