Related על חגיגה 2:1

They may not expound upon the subject of forbidden relations before three but they may expound before two, and they may not expound concerning the Work of Creation before two but they may expound before one (Chag. 2:1). They may not expound on the [Work of] the Chariot before one unless he is wise and understands of his own accord. A story of R. Yochanan ben Zakkai who was riding on his donkey and Rabbi Ele'azar ben Arakh was riding behind him. He said to him: "Rabbi, teach me one chapter concerning the Work of the Chariot. He said to him: "No. Have I not said to you from the beginning that one does not teach concerning the [Work of] the Chariot before one unless he is wise and understands of his own accord?" He said to him: "Now give me permission to expound before you." He said to him: "Speak!" Rabbi Ele'azar ben Arakh began to expound on the Work of the Chariot. Rabbi Yochanan ben Zakkai alighted from his donkey and wrapped himself in his tallit and the two of them sat on an outcropping of rock under an olive tree and he presented the subject before him. Rabbi Yochanan stood up and kissed him and said: "Blessed is Adonai, God of Israel, who gave a son to Abraham our father that knows and understands how to expound on the glory of our Father in Heaven!" There are those suited to expound and who are not suited to fulfill, those who are suited to fulfill and who are not suited to expound. Ele'azar ben Arakh is suited to expound and to fulfill. Happy is Abraham our father, that Ele'azar ben Arakh, who came forth from your loins, knows and understands well enough to expound on the glory of our Father in heaven!" Rabbi Yosi bar R. Yehuda says: "Rabbi Yehoshua expounded before R. Yochanan ben Zakkai, Rabbi Akiva expounded before Rabbi Yehoshua and Chananiah ben Chachinai expounded before Rabbi Akiva."