I found this reference online (italics mine):
Pirke deRabbi Eliezer 42
Rabbi Nechunia, son of Hakanah, said: Know the power of repentance. Come and see from Pharaoh, king of Egypt, who rebelled most grievously against the Rock, the Most High, as it is said, Who is the Lord, that I should hearken unto his voice? (Ex 5: 2). In the same terms of speech in which he sinned, he repented, as it is said, Who is like you, a Lord, among the mighty?(ibid.15:11). The Holy One, blessed be He, delivered him from amongst the dead. Whence (do we know) that he died? Because it is said, For now I had put forth my hand, and struck you (ibid. 9:15). He went and ruled in Nineveh. The men of Nineveh were writing fraudulent deeds, and everyone robbed his neighbor, and they committed sodomy, and such like wicked actions. When the Holy One, blessed be He, sent for Jonah, to prophesy against (the city) its destruction, Pharaoh hearkened and arose from his throne, rent his garments and clothed himself in sackcloth and ashes, and had a proclamation made to all his people, that all the people should fast for two days, and all who did these (wicked) things should be burnt by fire. What did they do? The men were on one side, and the women on the other, and their children were by themselves; all the clean animals were on one side, and their offspring were by themselves. The infants saw the breasts of their mothers, they wished to have suck, and they wept. The mothers saw their children, (and they wished) to give them suck. By the merit of children more than twelve hundred thousand men (were saved), as it is said, And should not I have pity on Nineveh, that great city; wherein are more than six score thousand persons that cannot discern between their right hand and their left hand; and also much cattle? (Jonah 4:11); And the Lord repented of the evil, which he said he would do unto them (ibid. 3:10). For forty years was the Holy One, blessed be He, slow to anger with them, corresponding to the forty days during which He had sent Jonah. After forty years they returned to their many evil deeds, more so than their former ones, and they were swallowed up like the dead, in the lowest Sheol, as it is said, Out of the city of the dead they groan (Job 24:12).
But I found it here as chapter 43.