Midrasz do Diwrej ha-jamim II 29:11

And thus do you find with Chizkiyahu, king of Judah, who taught all of Torah to Israel and called them "sons," viz. (II Chronicles 29:11) "My sons, now be not lax." And just as disciples are called "sons," so is the master called "father," viz. (II Kings 2:12) "And Elisha saw (Eliyahu being borne aloft) and he cried out 'My father, my father — the chariot of Israel and its riders!'" And he saw him no more, and he took hold of his own garments, etc." And it is written (Ibid. 13:14) "And Elisha fell ill with the illness of which he would die, and Yoash the king of Israel came down to him and wept before him, crying 'My father, my father!'"