Assalaamu alaykum. Please read the whole question. May Allaah reward you. There is a verse that says that the sun goes down in the sea in soorah 18. Now, I know that this means that it apeared to be doing so to Thul Qarnyan, and I saw someone even prove this from the Arabic without the possibility of doubt, but my question is the following: I heard a disbeliever say that big scholars like At-Tabari and Az-Zamakhshari believed that this verse meant that the sun literaly went down in the sea. I know that this (interpretation) is wrong, so I do not need an answer to this; my question, however, is: Did these scholars really say that or have they been misquoted?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

At-Tabari and Az-Zamakhshari did not state that the verse means that the sun sank into the sea, nor did they state that they believed that. Rather, they interpreted the verse as to mean that the sun sank into a spring that was described as 'hami'ah' (muddy) according to one mode of recitation and 'haamiyah' (of hot water) according to the other reported mode of recitation. They cited on the authority of Kaʻb Al-Ahbaar that the verse means that the sun sank into a muddy spring. It is well-known that Kaʻb used to narrate from the 'Israa'eeliyyaat' (body of narratives originating from Jewish and Christian traditions). It is also known that the Quranic commentators do not believe in the authenticity of the Israa'eeliyyaat nor reject them as long as they do not contradict the Islamic sharee'ah or are proven inauthentic by evidence in the Quran or Sunnah.

Quran commentators advised that the verse means that the sun disappeared from the vision of the beholder, not that it literally disappeared into the water.

Al-Fakhr Ar-Raazi wrote:

“It has been reported that the sun sets into a muddy spring of  abundant water. However, this is rather far-fetched and unlikely. This is because when we see an eclipse of the moon, those living in the West say that it happened at the beginning of the night, and those in the East say that it happened at the beginning of the day; so we know that the beginning of the night in the West is the beginning of the day in the East, and the time that is the beginning of the night for us is the ʻAsr time for a place, the Dhuhr time for a second place; the forenoon for a third one, sunrise at a fourth place, and midnight at a fifth place, and such is the certain case after investigating the issue. Moreover, we also know that the sun has evidently arisen in all these places, so for us to say that it (literally) sets in the spring of water is a contradiction to certain observable information, and the Word of Allaah is above such accusations, so there is no choice but to resort to the interpretation that we mentioned.”

Tafsir Al-Jalaalayn reads, “...The verse means that when he reached the place where the sun sets, he found it setting in a (muddy) spring described as 'Hami'ah', which means black mud. The sun’s setting in the spring is from the eye’s point of view, for the sun is larger than Earth. Allaah said (what means), {And he found near it a people}, i.e. near that spring ...”

As-Saʻdi wrote, “Allaah, The Exalted, blessed him with the means to reach the place of the setting of the sun, where he saw it setting as if it was setting into a black, muddy, hot spring according to the perception of his eyes, in the same way that the beholder would see the sun setting into the sea if the sea was between him and the western horizon, although the is very high in the sky.” [Tayseer Ar-Rahmaan fi Tafseer Kalaam Al-Mannaan]

Allaah knows best.