Sahih Muslim | Book 1 | Hadith Number 299

Narrated by Abu Dharr
Abu Dharr reported: 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.