In The Legends of the Jews, Ginzberg relates this story:
The first time Adam witnessed the sinking of the sun be was also seized with anxious fears. It happened at the conclusion of the Sabbath, and Adam said, "Woe is me! For my sake, because I sinned, the world is darkened, and it will again become void and without form. Thus will be executed the punishment of death which God has pronounced against me!" All the night he spent in tears, and Eve, too, wept as she sat opposite to him. When day began to dawn, he understood that what he had deplored was but the course of nature, and be brought an offering unto God, a unicorn whose horn was created before his hoofs, and he sacrificed it on the spot on which later the altar was to stand in Jerusalem.
Does anyone know an earlier source for this story?