The correct way to describe the sun is that it “prostrates beneath the Throne” and not that it “sets in a spring of warm water”
Question

                It is narrated by Abu Dawood in as-Sunan (3991) that Abu Dharr (may Allah be pleased with him) said: I was with the Messenger of Allah (blessings and peace of Allah be upon him), riding a donkey, and the sun was setting. He said: “Do you know where this (sun) goes?” I said: Allah and His Messenger know best. He said: “It sets in a spring of warm water (innaha taghrubu fi ‘aynin hami’ah).” Is this hadeeth saheeh? If it is, then what does it mean? I have heard some non-Muslims using this to stir up doubts about Islam and cast aspersions on it.
Praise be to Allah.
    Firstly: 

    The hadeeth of Abu Dharr (may Allah be pleased with him) 
    which describes the place where the sun goes was narrated via Ibraaheem ibn 
    Yazeed at-Taymi, from his father, from Abu Dharr, from the Prophet 
    (blessings and peace of Allah be upon him. 

    It was narrated from Ibraaheem at-Taymi by al-Hakam ibn 
    ‘Utaybah, al-A‘mash, Fudayl ibn Ghazwaan, Haroon ibn Sa‘d, and Moosa ibn 
    al-Musayyab, as was stated by Ibn Mandah in al-Eemaan (2/926) 

    They narrated it via two isnaads: 

    The first isnaad: 

    It was narrated by al-A‘mash, Yoonus ibn ‘Ubayd, Moosa ibn 
    al-Musayyab and Haroon ibn Sa‘d. In this report it says that the sun goes 
    and prostrates beneath the Throne or before its Lord; there is no mention of 
    a spring of warm water. It appears in several versions, of which we will 
    mention one here: 

    It was narrated that Abu Dharr (may Allah be pleased with 
    him) said: The Prophet (blessings and peace of Allah be upon him) said to 
    Abu Dharr when the sun set: “Do you know where it goes?” I said: Allah and 
    His Messenger know best. He said: “It goes and prostrates beneath the 
    Throne, then it asks for permission (to rise) and permission is given to it. 
    Soon it will prostrate, but it will not be accepted from it, and it will ask 
    for permission (to rise) but permission will not be given to it; it will be 
    said to it: “Go back to where you came from.’ So it will rise from its place 
    of setting, and that is what Allah, may He be glorified, refers to in the 
    verse (interpretation of the meaning): “And 
    the sun runs on its fixed course for a term (appointed). That is the Decree 
    of the All-Mighty, the All-Knowing” [Yaa-Seen 
    36:38]. 

    It was narrated via A‘mash by al-Bukhaari in his Saheeh 
    (no. 3199, 4802 and 7424); Muslim in his Saheeh (no. 159); Abu Dawood 
    at-Tayaalisi in al-Musnad (1/368); Ahmad in al-Musnad (35/282, 
    429); at-Tirmidhi in as-Sunan (no. 2186, 3227 – he said: It is hasan 
    saheeh; an-Nasaa’i in as-Sunan al-Kubra (10/229); at-Tabari in 
    Jaami‘ al-Bayaan, 20/516); al-Bazzaar in al-Bahr al-Zukhaar 
    (9/409); Abu ‘Awaanah in al-Mustakhraj (1/100-101); at-Tahhaawi in 
    Sharh Mushkil al-Athaar (1/254); Ibn Hibbaan in his Saheeh 
    (14/24); al-Qutay‘i in Juz’ al-Alf Dinar (p. 183); Abu Nu‘aym in 
    al-Musnad al-Mustakhraj ‘ala Saheeh Muslim (1/222); al-Baghawi in 
    at-Tafseer (4/14); Abu ash-Shaykh in al-‘Azamah (4/1192); Ibn 
    Mandah in al-Eemaan (2/924) and at-Tawheed (1/134, 135); Ibn 
    Bashraan in al-Amaali (p. 159); al-Bayhaqi in al-Asma wa as-Sifaat 
    (2/273); Ibn ‘Asaakir in al-Mu‘jam (2/1015); Abu Nu‘aym in Hilyat 
    al-Awliya’ (4/216) and others. 

    It was narrated via Yoonus ibn ‘Ubayd by Imam Muslim in his
    Saheeh (no. 159); an-Nasaa’i in as-Sunan al-Kubra (10/96); 
    at-Tabari in Jaami‘ al-Bayaan (12/249); Abu ‘Awaanah in 
    al-Mustakhraj (1/100); Abu al-‘Abbaas as-Siraaj in Hadeeth as-Siraaj 
    (3/258); Ibn Hibbaan  in his Saheeh (14/21); Abu ash-Shaykh in 
    al-‘Azamah (4/1189); Ibn Mandah in al-Eemaan (2/925, 926) and in
    at-Tawheed (1/136); Abu Nu‘aym in al-Mustakhraj (1/221). 

    It was narrated via Moosa ibn al-Musayyab by: Abu ash-Shaykh 
    al-Asbahaani in al-‘Azamah (4/1188);

    And via Haroon ibn Sa‘d by at-Tabaraani in al-Mu‘jam 
    al-Kabeer (4/373); Abu ash-Shaykh al-Asbahaani in al-‘Azamah 
    (4/1190). 

    Abu Nu‘aym (may Allah have mercy on him) said: 

    This is a saheeh hadeeth on which they (the hadeeth scholars) 
    are agreed, from the hadeeth of al-A‘mash from Sufyaan ath-Thawri and 
    others. It was narrated from at-Taymi al-Hakam ibn ‘Utaybah, Fudayl ibn 
    ‘Umayr, Haaroon ibn Sa‘d, Moosa ibn al-Musayyab, Habeeb ibn Abi al-Ashras, 
    and from Yoonus ibn ‘Umayd among the Basris.

    End quote from Hilyat al-Awliya’ (4/216) 

    The second isnaad: 

    It was narrated by al-Hakam ibn ‘Utaybah only, from Ibraaheem 
    at-Taymi; in this report it says that the sun sets in a spring of warm 
    water. 

    It was narrated that Abu Dharr said: 
    I was riding behind the Messenger of Allah 
    (blessings and peace of Allah be upon him) when 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 warm water (innaha 
    taghrubu fi ‘aynin haamiyah).”

    Narrated by Ahmad in al-Musnad (35/363); Abu Dawood in
    as-Sunan (no. 4002); Hafs ad-Doori in Juz’ Qiraa’aat an-Nabi sall 
    Allahu ‘alayhi wa sallam (p. 123); al-Bazzaar in al-Bahr al-Zakhaar 
    (9/407); as-Siraaj in his Hadeeth (3/258); al-Haakim in 
    al-Mustadrak (2/267) – he said: This is a hadeeth with a saheeh isnaad, 
    although they (al-Bukhaari and Muslim) did not narrate it. All of them 
    narrated it via Yazeed ibn Haaroon, from Sufyaan ibn Husayn, from al-Hakam. 

    Al-Bazzaar said: We do not know that anyone narrated it from 
    al-Hakam ibn ‘Utaybah, from Ibraaheem, from his father, from Abu Dharr 
    except Sufyaan ibn Husayn. Yoonus ibn ‘Ubayd narrated it from Ibraaheem 
    at-Taymi, Sulayman al-A‘mash and Haroon ibn Sa‘d. End quote. 

    Secondly: 

    From the above it is clear that the version of the hadeeth of 
    Abu Dharr which is most likely to be correct is the first one, in which it 
    says: “It goes and prostrates beneath the Throne”, in which it makes no 
    mention of it setting “in a spring 
    of warm water (innaha taghrubu fi ‘aynin haamiyah).” 
    This is based on several factors: 

    ~1~

    This wording is that on which the majority of narrators and 
    those with the best memories agreed upon, in the report from Ibraaheem 
    at-Taymi. 

    ~2~

    The second version (“in a spring of warm water”) was narrated 
    only by al-Hakam ibn ‘Utaybah from Ibraahem, and al-Hakam – although he was 
    trustworthy – was described by an-Nasaa’i as mudallis [i.e., he used vague 
    wording to give a wrong impression] in Dhikr al-Mudalliseen (no. 11). 
    Ibn Hibbaan said: He used to engage in tadlees. End quote from 
    ath-Thiqaat (4/144). None of those who narrated his hadeeth said that he 
    clearly stated that he heard it from the previous narrator; rather they all 
    said that his report included the word ‘an (from – instead of ‘I 
    heard’, etc). Ibn Hajar mentioned him in Maraatib al-Mudalliseen in 
    the second rank of mudalliseen (p. 30), who are the ones whose tadlees the 
    imams (leading scholars) tolerated and from whom they narrated reports that 
    they regarded as sound, because of their prominence and because they rarely 
    engaged in tadlees  compared to others, such as ath-Thawri; or (it was 
    tolerated) because they did not use engage in tadlees except when narrating 
    from a trustworthy source, such as Ibn ‘Uyaynah. The fact that this report 
    is contrary to the report narrated by the trustworthy narrators confirms the 
    possibility that tadlees occurred in this hadeeth in particular. 

    ~3~

    The first version was narrated by both al-Bukhaari and 
    Muslim, whereas the second version was not narrated by these two shaykhs. 
    That is because they thought the first version was more likely to be correct 
    than any other. Undoubtedly the reports in the books of al-Bukhaari and 
    Muslim take precedence, according to the critics and scholars, than those in 
    other books and Musnads. 

    ~4~

    There is a great difference between the two versions. The 
    first one describes the sun as prostrating beneath the Throne, whether at 
    the time of sunset or at any other time, according to different versions of 
    the hadeeth. Prostration beneath the Throne is a matter of the unseen, and 
    no one knows how it is except Allah, may He be glorified and exalted. He, 
    may He be glorified and exalted, has told us that all created things 
    prostrate to Him, as He says (interpretation of the meaning):

    “See you not that to 
    Allah prostrates whoever is in the heavens and whoever is on the earth, and 
    the sun, and the moon, and the stars, and the mountains, and the trees, and 
    Ad-Dawab (moving living creatures, beasts, etc.), and many of mankind? But 
    there are many (men) on whom the punishment is justified”

    [al-Hajj 22:18].

    No one knows how this prostration is or when it happens or 
    its real nature except Allah, may He be glorified and exalted. Similarly, 
    the hadeeth of Abu Dharr is no different from this verse at all, and there 
    is nothing objectionable in it. 

    Al-Khattaabi (may Allah have mercy on him) said:

    It cannot be denied that it halts beneath the Throne, in a 
    manner that we cannot see or comprehend; rather this is speaking about a 
    matter of the unseen. So we should not disbelieve in it or ask how it 
    happens, because our knowledge cannot comprehend it.

    End quote from A‘laam al-Hadeeth Sharh Saheeh al-Bukhaari 
    (p. 1893) 

    With regard to the second version, “It sets in a spring of 
    warm water”, this is problematic, because the setting of the sun occurs when 
    its disc disappears below the horizon, and it sets on some people and rises 
    on others because of the movement of the earth around it. So it is not 
    possible that the sun, which is of immense size, could disappear into a 
    spring of warm water. With regard to the words of Allah, may He be exalted, 
    in the story of Dhu’l-Qarnayn (interpretation of the meaning): 
    “Until, when he 
    reached the setting place of the sun, he found it setting in a spring of 
    black muddy (or hot) water” [al-Kahf 18:86], 
    the commentators said that what is meant is to depict the scene that 
    appeared before him, for he saw it on the horizon of the sea, as if it were 
    sinking into it at the time of setting. The Holy Qur’an does not tell us 
    that this was something that happnened in a real sense; hence Allah, may He 
    be glorified, said “he found it setting” and He did not say “it was setting 
    in a spring of black muddy (or hot) water.” 

    Al-Haafiz Ibn Katheer (may Allah have mercy on him) said:

    That is, he way he saw the sun was as if it was setting in 
    the ocean, which is something that happens to everyone who stands on the 
    shore: he sees the sun as if it is setting into the sea, when in fact it 
    does not leave the celestial path on which it is firmly established. 
    

    End quote from Tafseer al-Qur’an al-Kareem (5/191) 

    To sum up: whoever casts aspersions on the Sunnah because of 
    this wording is ignorant of the precision of the hadeeth scholars in 
    examining different versions of the hadeeth and distinguishing between the 
    reports which are proven and those which are not proven. The fact that some 
    scholars regarded this report as saheeh is problematic; rather what they 
    intended was to class as saheeh the basis of the hadeeth, not to suggest 
    that this version is more correct than that which is proven in 
    as-Saheehayn: “It goes and prostrates beneath the Throne.” 

    In order to refute this specious argument, it is sufficient 
    to note that the scholars of Islam were unanimously agreed that the earth is 
    round and that all the celestial bodies are also round. It is on that basis 
    that night and day alternate, because of the movements and rotation of these 
    heavenly bodies. This is clear from many texts of the Qur’an and Sunnah. 

    Ibn Taymiyah (may Allah have mercy on him) said: 

    The heavenly bodies are round, according to the Muslim 
    scholars among the Sahaabah and those who followed them in guidance; that 
    was also proven from them with isnaads mentioned in the appropriate places. 
    In fact, more than one of the Muslim scholars narrated that there was 
    consensus among the Muslims on that point.

    End quote from Minhaaj as-Sunnah an-Nabawiyyah 
    (5/442) 

    The Muslims were ahead of others in knowledge of astronomy; 
    if the apparent meaning of this hadeeth contradicts that, then there would 
    not have been consensus on the contrary; but the correct view is that the 
    hadeeth does not contradict the scientific, astronomical facts. 

    And Allah knows best.