It was narrated that Abu Dharr said: “I was riding behind the Messenger of Allah while he was on a donkey, and the sun was setting. He said: ‘Do you know where this (sun) sets?’ I said: ‘Allah and his Messenger know best.’ He said: ‘IT SETS IN A SPRING OF WATER (fa innaha taghrubu fi ‘ainin hamiyah).’”
(SAHIH) (English Translation of Sunan Abu Dawud, Compiled by Imam Hafiz Abu Dawud Sulaiman bin Ash‘ath – From Hadith no. 3242 to 4350, ahadith edited and referenced by Hafiz Abu Tahir Zubair ‘Ali Za’i, translated by Nasiruddin al-Khattab (Canada), final review by Abu Khaliyl (USA) [Darussalam Publishers & Distributors, First Edition: July, 2008]
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The hadeeth is authentic.