Musnad Ahmad 937:170

Yazid narrated to us, Sufyan—meaning Ibn Husayn—narrated to us, from al-Hakam, from Ibrahim al-Taymi, from his father, from Abu Dharr, who said: I was with the Prophet, peace and blessings of Allah be upon him, on a donkey, and upon it was a saddlecloth or a velvet cloth. He said: And that was at the setting of the sun. He said to me: "O Abu Dharr, do you know where this sets?" I said: Allah and His Messenger know best. He said: "It sets in a hot spring. It proceeds until it falls prostrate before its Lord beneath the Throne. When the time for its emergence arrives, Allah permits it, so it emerges and rises. When He intends to make it rise from where it sets, He will restrain it, and it will say: 'O my Lord, my journey is far.' Then He will say to it: 'Rise from where you set.' That is when 'no soul will benefit from its faith.'"