Sahih Muslim, Chapter: Faith (Kitab Al Iman)

I entered the mosque and the Messenger of Allah (may peace be upon him) was sitting there. When the sun disappeared (from the sight) he said: O Abu Dharr! Do you know where it goes? He (the narrator) said: Allah and His Apostle know best. He (the Holy Prophet) said. Verily it goes and begs permission, for prostration (to Allah) and the permission is granted to it. Once it would be said: Return to the place whence you came, and then it would rise from its setting place. Then he, after the recitation of 'Abdullah recited it: And that is its appointed term.