Jerusalem Talmud, Tractate Hagiga, chapter 2, page 77

...When Rabbi Lazer ben Arach began [his discourse] on Ma'aseh Merkavah ['Workings of the Chariot,' i.e. the mystical experience], Rabbi Yochanan ben Zakai descended from the donkey. He said, it is not fitting that you hear the glory of my Acquirer and I ride on the donkey. They went and sat under a tree, and fire descended from heaven and surrounded them, and heavenly angels were jumping before them like those at a wedding joyous before the groom. An angel answered from the fire and said, 'According to your word, Elazar ben Arach—so are the Workings of the Chariot.' Immediately all the trees opened their mouths and said song, 'Then the trees of the forest will sing with joy' (Psalms ch. 96 and I Chronicles 16:33). When Rabbi Lazer ben Arach finished with the Workings of the Chariot, Rabbi Yochanan ben Zachai stood and kissed him on his head, and said 'Blessed is Hashem, the G-d of Abraham, Isaac, and Jacob, who gave to Abraham our father a wise son who knows how to expound the glory of our Father in Heaven...” [translation by Y. Neril]