Assalaamu alaykum, dear Scholar. I have a question. Did the prophet, sallallaahu ʻalayhi wa sallam, give permission to a group of Christians to do their prayer in the mosque. Is there a hadith narrating this? What is the authenticity of the hadith? Ibn Ishaq said, “Muhammad bin Ja`far bin Az-Zubayr said that the (Najran) delegation came to the Messenger of Allaah in Al-Madinah and entered his masjid wearing robes and garments after the Prophet had prayed the `Asr prayer. They accompanied a caravan of camels led by Bani Al-Harith bin Ka`b. The Companions of the Messenger of Allaah, sallallaahu ʻalayhi wa sallam, who saw them said that they never saw a delegation like them after that. When their time of worship came, they stood up to perform their worship in the Prophet’s masjid. The Messenger of Allaah, sallallaahu ʻalayhi wa sallam, said, 'Let them (worship),' and they prayed towards the east.” Please shed some light on this issue. May Allaah reward you.

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. 

Ibn katheer wrote:

“Ibn Is-haaq said, 'Muhammad ibn Jaʻfar ibn Az-Zubayr narrated that the (Najraan) delegation came to the Messenger of Allaah, sallallaahu ʻalayhi wa sallam, in Madeenah and entered his masjid wearing robes and garments after he had prayed the ʻAsr prayer. They accompanied a caravan of camels led by Bani Al-Haarith ibn Kaʻb. The Companions of the Messenger of Allaah who saw them said that they never saw a delegation like them after that. When their time of worship came, they stood up to perform their worship in the Prophet's masjid. He, sallallaahu ʻalayhi wa sallam, said, ‘Let them (worship),’ and they prayed towards the east.'” [Tafseer Ibn Katheer]

We could not find a chain of narration for this story except for this one on the authority of Ibn Is-haaq which has some narrators missing in its chain of transmission indicating its weakness. In any case, if this report is proven authentic, it does not indicate that it is permissible for the non-Muslims to enter the masjid and perform their prayers therein without restrictions. On the contrary, it is restricted to a clear benefit such as inviting them to Islam. In the story of Thumaamah when he was taken as a prisoner of war before his conversion to Islam, he was detained in the masjid and tied to one of its columns. The story was cited in Saheeh Al-Bukhari and Saheeh Muslim. Some scholars held that the reason that he was tied to a pillar in the masjid was to make him see the Muslims while performing the prayer and reciting the Quran.

Shaykh Al-Fawzaan said, “It is permissible to allow the non-Muslim to enter the masjid if there is benefit in this, such as making him see the prayer and recitation of the Muslims, as in the story of Thumaamah .”

Shaykh Muhammad ibn Ibraaheem Aal Ash-Shaykh wrote, “It is impermissible to allow them to enter the mosques, regardless of whether that be in Madeenah or elsewhere, unless there is a guaranteed benefit in allowing that. If there is a guaranteed benefit, then it is permissible, as was narrated in the story of the Christians of Najraan when they entered the mosque of the Prophet, sallallaahu ʻalayhi wa sallam, and their prayer was due, so they recited their prayers therein. This took place in the year of Delegations.”

Allaah knows best.