Jami` at-Tirmidhi 2186

Abu Dharr said:"I entered the Masjid at sunset, and the Prophet(s.a.w) was sitting. He said: 'O Abu Dharr! Do you know where this(sun) goes?' I said:'Allah and His Messenger know better.' He said: 'Indeed it goes to seek permission to prostrate, so it is permitted. And it is as if it has been said to it: "Rise from whence you came." So it shall rise from its setting place.' Then he recited: 'That is its fixed course."' He said: "That is the recitation of 'Abdulla bim Mas'ud