Rabbi Yochanan ben Zakkai was riding his donkey along a road when his student Rabbi Eleazar ben Arach asked for permission to expound the secrets of the Ma'aseh Merkavah, the mysteries of the divine chariot seen by the prophet Ezekiel. These are the deepest mysteries in the Jewish tradition. One does not discuss them lightly.
Rabbi Yochanan granted permission, but he did not proceed while mounted. He dismounted at once. He explained to his student that ministering angels might well be present to listen, and it was not fitting to ride a donkey in the presence of angels. He sat down on a stone beneath an olive tree. Then he said, "Begin."
Eleazar began to speak. Fire descended from heaven and surrounded them. The olive tree and the surrounding trees broke into song. When Eleazar finished, Rabbi Yochanan rose, kissed him on the head, and said, "Blessed is the Lord, God of Israel, who has given to Abraham our father a son like you."
When this story reached Rabbi Yehoshua and Rabbi Yose the priest, they wanted to experience the same thing. They sat together to expound the mysteries themselves. It was the seventeenth of Tammuz, a day heavy with destruction in Jewish memory. The sky above them clouded over. The ministering angels gathered like wedding guests coming to a feast.
They told Rabbi Yochanan what they had seen. He answered with the phrase that every student longs to hear. "Blessed are you, and blessed are my eyes which saw this. In a dream I saw you and me sitting together on Mount Sinai, and a voice from heaven called to us, 'Ascend, ascend, large palaces are ready for you, large couches are ready for you and your pupils.'" The Exempla preserves this scene from the tannaitic tradition also recorded in Chagigah (14b).