In Surah 18 of the Quran, Muhammad tells the story of Zul-Qarnain. In another article, entitled Muhammad's Theft of the Alexander Romance Completely Disproves Islam, it's demonstrated that this person is Alexander the Great, and the entire story is stolen from a popular legendary biography of him, which was definitely in circulation in Muhammad's time.
In the story, we are told that this man reached the place where the sun sets, and it set in a pool of murky water:
Surah 18:86 (Yusuf Ali, 1938)
86 Until, when he reached The setting of the sun, He found it set In a spring of murky water: Near it he found a people. We said: "O Ẓul-qarnain! Thou hast authority, either To punish them, or to treat them with kindness."
Surah 28:83-101 contains the complete story, and in it, we also learn that he found the place where the sun rises:
Surah 18:89-90 (Yusuf Ali, 1938)
89 Then followed he (another) way,
90 Until, when he came to the rising of the sun, he found it rising on a people for whom We had provided no covering protection against the sun.
I include the translator's name above because as we will see, some modern translators change the sunset passage to try and cover up a story that they know is false.
Another famous early English translation by Marmaduke Pickthall renders the passage accurately, as Yusuf Ali:
Surah 18:86 (Pickthall, 1930)
86 Till, when he reached the setting-place of the sun, he found it setting in a muddy spring, and found a people thereabout. We said: O Dhu'l-Qarneyn! Either punish or show them kindness.
This is how the passage in rendered by Arthur Arberry (1955), Abul Maududi (1972), Halali-Khan (2000's), Laleh Bakhtiar (2007), and Talal Itani (2012).
Now we'll look at the translation published by the Saudi Arabian government, Sahih International, to see what the dishonest translators do to the passage:
Surah 18:86 (Sahih International, 1997)
86 Until, when he reached the setting of the sun, he found it [as if] setting in a spring of dark mud, and he found near it a people. Allah said, "O Dhul-Qarnayn, either you punish [them] or else adopt among them [a way of] goodness."
Notice the "as if", which at least has been placed in brackets to let you know that it wasn't in the Arabic. Another version which does this is Haleem's (2004).
Other modern translators like Mustafa Khattab (2015) and Muhammad Asad (1980) don't even have the brackets. They simply insert the words "it appeared to him to be" (setting in a dark spring) into the passage as if it was in the Arabic, when it isn't. This is obviously in order to make it seem like this is just from the man in the story's misgiven perspective.
However, it's not from the man's perspective, obviously, because he's spoken about in the third person. God is ostensibly the one speaking, and it refers to the man with "he", not a first-person pronoun. This is Allah telling us what the man did, not the man describing what he saw.
This "misgiven perspective" rebuttal is really the only Muslim response to this passage. When you point out that the Quran says plainly that the man found the place where the sun sets, and that it sets in a murky pond, Muslims will hasten to point out that he is only telling the story from Zul-Qarnain's misgiven perspective.
However, beyond simply reading the passage, we can further know that this isn't true because of a Sahih narration from Sunan Abu-Dawud:
Sunan Abu-Dawud 4002
Narrated Abu Dharr:
I was sitting behind the Messenger of Allah (ﷺ) who was riding a donkey while the sun was setting. He asked: Do you know where this sets? I replied: Allah and his Apostle know best. He said: It sets in a spring of warm water.
Above, we have a narration from one of Muhammad's companions which makes it clear that Muhammad himself believed that the sun set in a pool of water, which is evidence that Surah 18:86 is not speaking about Zul-Qarnain's perspective. Muhammad believed the story himself, and it's narrated from his perspective, which is why the sun setting in water is stated as a fact with zero indication anywhere to the contrary.
And again, lest there be any doubt that Surah 18 refers to a physical location where the sun sets, please reference the aforementioned article on the Alexander Romance, where that is made even more abundantly clear.
Muhammad also said things which indicated a clear-cut geocentric view of the universe. Geocentrism is the doctrine that the earth is fixed, and the sun and other heavenly bodies move in relation to it, rather than the earth being in orbit. It's not always coincidental with the flat earth, but I will argue that it is in Muhammad's case.
As this kind of accusation is often levied against the Bible, I have been careful to examine context, and allow for figurative language. Yet, I still think the following teaches an indisputable flat-earth geocentrism:
Sahih al-Bukhari 3199
Narrated Abu Dhar:
The Prophet (ﷺ) asked me at sunset, "Do you know where the sun goes (at the time of sunset)?" I replied, "Allah and His Apostle know better." He said, "It goes (i.e. travels) till it prostrates Itself underneath the Throne and takes the permission to rise again, and it is permitted, and then (a time will come when) it will be about to prostrate itself but its prostration will not be accepted, and it will ask permission to go on its course but it will not be permitted, but it will be ordered to return whence it has come, and so it will rise in the west. And that is the interpretation of the Statement of Allah: "And the sun Runs its fixed course For a term (decreed). That is The Decree of (Allah) The Exalted in Might, The All-Knowing." (Surah 36:38)
This is also narrated in Sahih al-Bukhari 4802-03, 7433, and Sahih Muslim 159a-d, and it is reiterated that this is Muhammad's commentary on Surah 36:38.
The above does not make sense, because the sun does not "go" anywhere at sunset. The sun is where it is in space, and the earth turns once per day, while orbiting around the sun. Therefore, it is always sunset, somewhere on earth. How could the sun "go till it prostrates under Allah's throne" at sunset, when the sun does not move, and it is always sunset somewhere on earth?
If Allah's throne moved to always be over the sun to account for the globe-shaped earth, then how would this language make any sense? In that situation, the sun would be in constant prostration, and so Muhammad's description of the sun "going" and prostrating under Allah's throne at night would make no sense.
Taking the above as honestly and fairly as I know how, I believe that this presents a situation in which the earth is flat, and during night time, the sun goes to somewhere unseen for all of earth, where Allah's throne is, before getting permission to return to the other side of the flat earth, completing its course. It uses language in which that scenario makes sense. It does not use language which would suggest an orbiting, spherical planet.
Similarly, he indicated this flat-earth geocentric view when forbidding praying at noon:
Sunan an-Nasai 572
Abu Yahya Sulaim bin 'Amir, Damrah bin Habib and Abu Talhah Nu'aim bin Ziyad said:
"We heard Abu Umamah Al-Bahili say: 'I heard 'Amrah bin 'Abasah say: I said: 'O Messenger of Allah, is there any moment which brings one close to Allah than another, or any moment that should be sought out for remembering Allah? He said: 'Yes, the closest that the Lord is to His slave is in the last part of the night, so if you can be among those who remember Allah at that time, then do so. For prayer is attended and witnessed (by the angels) until the sun rises, then it rises between the two horns of the Shaitan, that is the time when the disbelievers pray, so do not pray until the sun had risen to the height of a spear and its rays have disappeared.
Then prayer is attended and witness (by the angels) until the sun is directly overhead at midday, and that is the time when the gates of Hell are opened and it is stoked up. So do not pray until the shadows appear. Then prayer is attended and witnessed (by angels) until the sun sets, and it sets between the horns of a Shaitan, and that is the time when the disbelievers pray.'"
Above, Muhammad says not to pray at midday, because that's when "the gates of Hell are opened, and it is stoked up". The problem with that is, it's always midday, somewhere on earth. So, are the gates of Hell always open?
Notice also that he says "the sun rises between the two horns of Satan", and therefore you may not pray at sunrise. This is also narrated concisely in Sahih Muslim:
Sahih Muslim 828b
Ibn 'Umar reported Allah's Messenger (ﷺ) as saying:
Do not intend to observe prayer at the time of the rising of the sun nor at its setting, for it rises between the horns of Satan.
The fact that the sun rises between the horns of Satan is further narrated Sunan ibn-Majah 1253, Sunan abu-Dawud 1277, and Sunan an-Nasai 572.
How does that make sense, given that it is always sunrise, somewhere on earth?
All of these statements indicate that Muhammad believed that sunrise/sunset were single events for all of humanity, once per day. They do not use language which demonstrates an understanding that there is a continual state of sunrise and sunset, depending on the rotation of the earth.
The story of the sun setting in a muddy spring is a scientific error in the Quran. Muhammad's other teachings about the sun demonstrate a belief in a flat earth. I do not find the Muslim rebuttals emphasizing "perspective" compelling, because these passages are clear, and clearly teach what they appear to be teaching to anyone without an agenda to defend Muhammad.