Yusuf bin Al-Husain is the narrator of this story:
I was with Dhi-an-Nun on the edge of a stream, when we saw an extraordinarily large lizard crawling along the side of the stream. Something strange yet wonderful then happened. A frog surfaced from the stream, the lizard mounted its back, and the frog then swam to the other side of the stream, carrying its passenger on its back.
Dhi-an-Nun said, “There is something interesting about that lizard let us go and follow it.” We crossed the stream (which wasn’t so wide in width), and on the other side, we saw a drunken man who had passed out. Suddenly, a snake appeared before the prone body of the drunkard. It slithered its way up to the man’s navel to his chest, making a direct course to one of his ears.
The lizard we saw earlier then sprang into action. It jumped onto the snake and struck it violently, splitting its body into two. Returning to the edge of the stream, the lizard then mounted the frog again, and the two crossed to the other side of the stream, though both were still visible to us. Dhi-an-Nun woke up the drunkard, who slowly opened his eyes and became conscious of what was happening around him. Dhi-an-Nun said, “O young man, look how Allah has saved you! When this snake came to kill you, that lizard came and killed it. He then explained the entire story to the young man.
The young man stood up and exclaimed: “O my lord, this is how you deal with one who disobeys you, then what will your mercy be like for the one who obeys you?” He then betook himself to the desert, vowing to dedicate his life to worship and to never return to the evils of the city.
The above is taken from “The Stories of Repentence” published by dar-us-salam