Masjid Nimrah is located in Waadi Urana (Urana Valley). When the Prophet (ﷺ) was at Arafat on his farewell Hajj on the 9th of Dhul Hijjah 10 AH he camped here.
After midday he delivered his famous sermon in the Urana Valley while seated on his camel, after which he led the salah. More than 100,000 Sahabah accompanied him on this Hajj....
In the second century of Islam, Masjid Nimrah was built at the spot where the Prophet (ﷺ) delivered the sermon and led the salah. Because Waadi Urana, where the sermon was delivered is outside the boundaries of Arafat, that section of the masjid is naturally also outside the boundaries.
When the masjid was extended afterwards, it then became divided into two sections; the front section which was the location of the original masjid being outside Arafat and the back section being within the boundaries. After renovations, signboards were erected inside the masjid to inform people about this matter so that after performing their Zuhr and Asar salahs in congregation, they could either move to the back of the masjid or outside to spend the rest of their time in Arafat. Should a person spend all his time from midday to sunset in the front section of the masjid, their stay at Arafat will be invalid and Hajj will therefore be incomplete.