Sahih Muslim Book - 6, Hadith - 1826

وَحَدَّثَنِى مُحَمَّدُ بْنُ حَاتِمٍ حَدَّثَنَا بَهْزٌ حَدَّثَنَا وُهَيْبٌ حَدَّثَنَا مُوسَى بْنُ عُقْبَةَ قَالَ سَمِعْتُ أَبَا النَّضْرِ عَنْ بُسْرِ بْنِ سَعِيدٍ عَنْ زَيْدِ بْنِ ثَابِتٍ أَنَّ النَّبِىَّ -صلى الله عليه وسلم- اتَّخَذَ حُجْرَةً فِى الْمَسْجِدِ مِنْ حَصِيرٍ فَصَلَّى رَسُولُ اللَّهِ -صلى الله عليه وسلم- فِيهَا لَيَالِىَ حَتَّى اجْتَمَعَ إِلَيْهِ نَاسٌ. فَذَكَرَ نَحْوَهُ وَزَادَ فِيهِ « وَلَوْ كُتِبَ عَلَيْكُمْ مَا قُمْتُمْ بِهِ ».

It was narrated from Zaid bin Thabit that the Prophet (s.a.w) sectioned off an area using a reed mat in the Masjid, and the Messenger of Allah (s.a.w) prayed in it for several nights, until people gathered to join him... and he mentioned a similar Hadith (as no. 1825), and added: "If it were made obligatory for you, you would not be able to do it."