Midrash su II Re 21:1

Our Rabbis were taught: When R. Eliezer became sick four elders entered to make him a sick call: R. Tarphon, R. Joshua, R. Elazar b. Azaryah, and R. Akiba. "You are better to Israel than drops of rain," exclaimed R. Tarphon, "for the latter are only in this world, you, our master, are in both, in this and in the world to come." To which R. Joshua answered: "You are better to Israel than the planet of the sun, which is only in this world, while you, our master, are in both, in this world and in the world to come." And R. Elazar b. Azaryah added, saying: "You are better to Israel than a father and mother, who are only in this world, while you, our master, are in both, in this world and in the world to come." R. Akiba then exclaimed: "Dear are sufferings [as a Divine trial]!" Whereupon R. Eliezer said: "Support me, and I will bear the statement of Akiba my disciple, who says, 'Dear are sufferings.' " "Akiba," said he, "whence do you know this?" And he answered: "I interpret the following passage (II Kings, 21, 1) Menasseh was twelve years old when he began to reign, and he reigned fifty and five years in Jerusalem … and he did that which was evil in the sight of the Lord. (Ib. b) It is also written (Prov. 25, 1) "These also are the proverbs of Solomon, which the men of Hezekiah, the king of Judah, copied out. How is it possible that Hezekiah taught the law to the whole world, but not to his son Manasseh? It must then be said that of all the troubles which Hezekiah has troubled himself, he did not succeed in bringing him back to the right way, but suffering did it, as it is said (II Chron. 33, 10-14) And the Lord spoke to Manasseh, and to his people; but they gave no heed. Wherefore the Lord brought upon them the captains of the host of the king of Asyria, who took Manasseh with hooks, and bound him with fetters, and carried him to Babylon. And it is written further, And when he was in distress, he besought the Lord his God, and humbled himself greatly before the God of his fathers. And he prayed unto Him; and He was entreated by him, and heard his supplication, and brought him back to Jerusalem into his kingdom. Then Manasseh knew that the Lord He was God. From this you can learn that sufferings are dear."

R. Akiva: "It is written (II Chronicles 33:1) 'Menasheh was twelve years old when he began to reign, and he reigned fifty-five years in Jerusalem.' And it is written (Proverbs 25:1) 'These, too, are the proverbs of Solomon, which were copied down by the servants of Chezkiah king of Judah.' Now would it enter your mind that Chizkiyahu taught Torah to all of Israel but not to Menasheh his son? But (it is to be understood that) of all the Torah he taught him and all the toil he invested in him naught availed him but afflictions, as it is written (II Chronicles 33:10-13) 'And the L-rd spoke to Menasheh and his people, but they did not listen. And the L-rd brought against them the officers of the king of Ashur's army. And they caught Menasheh with hunting hooks, and bound him in chains and led him to Bavel. And in his suffering he besought the L-rd his G-d, and he humbled himself greatly before the G-d of his fathers. And he prayed to Him, and He was entreated of Him, and He heard his supplication, and He returned him to Jerusalem to his kingdom" — whence we derive that afflictions are beloved.