Assalamualaikum ustaz. Is it permissible for a non-Muslim to clean a mosque, especially in the recent flood situation that occurred in our country? Thank you.
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
We start with Allah SWT’s statement:
يَا أَيُّهَا الَّذِينَ آمَنُوا إِنَّمَا الْمُشْرِكُونَ نَجَسٌ فَلَا يَقْرَبُوا الْمَسْجِدَ الْحَرَامَ بَعْدَ عَامِهِمْ هَٰذَا
“O you who have believed, indeed the polytheists are unclean, so let them not approach al-Masjid al-Haram after this, their [final] year.” 
The majority of scholars stated najis in the above verse means filth and uncleanliness in terms of their faith of non-Muslims that should be avoided and not their bodies. They argued that Islam sanctions that marrying the People of the Book is permissible and if their sweat is najis, then surely their husbands wouldn’t be safe from this najis. The majority of scholars also answered that the Prophet PBUH once performed ablution using the container of the polytheists, tied a polytheist captive in a mosque and ate cheese from the Catholic country and ate a lamb dish as a gift from a Jew.  For further explanation on this matter, you can refer to our previous article titled: #230: Non-Muslims Aren’t Najis. 
Non-Muslims Entering Mosques
The question asked is closely related to the ruling of non-Muslims entering mosques. This matter was mentioned in a hadith from Abu Hurairah RA, where he said:
بَعَثَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ خَيْلًا قِبَلَ نَجْدٍ، فَجَاءَتْ بِرَجُلٍ مِنْ بَنِي حَنِيفَةَ يُقَالُ لَهُ: ثُمَامَةُ بْنُ أُثَالٍ، فَرَبَطُوهُ بِسَارِيَةٍ مِنْ سَوَارِي المَسْجِدِ
“Allah’s Messenger (ﷺ) sent some horse men to Najd and they brought a man called Thumama bin Uthal from Bani Hanifa. They fastened him to one of the pillars of the mosque.” 
Imam al-Nawawi stated that this hadith shows the permissibility of tying a prisoner of war and arresting them, as well as the permissibility of non-Muslims entering the mosque. However, according to madhhab al-Syafi’I the permissibility of non-Muslims entering a mosque is subject to the permission is given by the Muslims, regardless of whether the non-Muslim is from the People of the Book or not. Whilst, Umar bin ‘Abd al-‘Aziz, Qatadah and Malik are of the opinion that non-Muslims shouldn’t enter a mosque. Whereas Abu Hanifah permits this matter for those who are from the People of the Book and not for others.
Next, the verse of the Quran that states the najis status of the polytheists and the prohibition against them from approaching the Masjidil Haram is specific only to Masjidil Haram. Hence, it is impermissible for them – non-Muslims – from ever entering the Masjidil Haram. 
Non-Muslims cleaning mosques
Coming back to the question asked, in our opinion, non-Muslims are permitted to enter and clean a mosque, but on the condition that they must first obtain permission from the Muslims – which is the mosque-s authority or management.
Furthermore, Syeikh Muhammad Soleh  was once asked regarding the ruling of a non-Muslim entering a mosque for repair or renovation works. He answered by saying that it is fine for a non-Muslim to enter a mosque to repair anything in it. However, one should be cautious of him. In other words, we must appoint a Muslim musyrif – to supervise the work being done. Then, he concluded that it is fine for them to enter the mosque for a certain maslahat. However, if the purpose is just to sit in there, then this is impermissible. Although, if he enters the mosque to drink some water from in the mosque, then this is fine. 
According to the above arguments and discussion, in our opinion, a non-Muslim is permissible to enter a mosque to clean it, but it is on the condition that he must first obtain permission from the mosque’s management. The reason is there is a clear benefit for the mosque and Muslims, especially after a huge flood that destroyed properties and places including mosques.
Thus, Muslims should welcome this good intention – which includes the help from non-Muslims – in helping to repair and clean the affected mosques and it should be supervised by the mosque’s authorities who are Muslims, as to maintain and protect the purity and honour of the mosque and Quran from the damaged mashaf or others by cleaning and disposing of them following syarak guidelines.
May Allah SWT grant us all a clear understanding in this religion. Amin.
 Surah al-Taubah: 28
 See Nail al-Autar, 1/35.
 Riwayat al-Bukhari (469); Muslim (1764)
 See al-Minhaj Syarh Sahih Muslim, 12/87.
 He was a member of Hai’ah Kibar al-‘Ulama ‘and a lecturer at al-Imam Muhammad bin Su’ud al-Islamiyyah University.
 See https://ar.islamway.net/fatwa/76039/ حكم-دخول-الكافر-المسجد