Sahih Bukhari Volume 1, Book 10, Hadith 729

حَدَّثَنَا مُحَمَّدٌ، قَالَ أَخْبَرَنَا عَبْدَةُ، عَنْ يَحْيَى بْنِ سَعِيدٍ الأَنْصَارِيِّ، عَنْ عَمْرَةَ، عَنْ عَائِشَةَ، قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّي مِنَ اللَّيْلِ فِي حُجْرَتِهِ، وَجِدَارُ الْحُجْرَةِ قَصِيرٌ، فَرَأَى النَّاسُ شَخْصَ النَّبِيِّ صلى الله عليه وسلم فَقَامَ أُنَاسٌ يُصَلُّونَ بِصَلاَتِهِ، فَأَصْبَحُوا فَتَحَدَّثُوا بِذَلِكَ، فَقَامَ لَيْلَةَ الثَّانِيَةِ، فَقَامَ مَعَهُ أُنَاسٌ يُصَلُّونَ بِصَلاَتِهِ، صَنَعُوا ذَلِكَ لَيْلَتَيْنِ أَوْ ثَلاَثَةً، حَتَّى إِذَا كَانَ بَعْدَ ذَلِكَ جَلَسَ رَسُولُ اللَّهِ صلى الله عليه وسلم فَلَمْ يَخْرُجْ، فَلَمَّا أَصْبَحَ ذَكَرَ ذَلِكَ النَّاسُ فَقَالَ ‏"‏ إِنِّي خَشِيتُ أَنْ تُكْتَبَ عَلَيْكُمْ صَلاَةُ اللَّيْلِ ‏"‏‏

Narrated By 'Aisha : Allah's Apostle used to pray in his room at night. As the wall of the room was LOW, the people saw him and some of them stood up to follow him in the prayer. In the morning they spread the news. The following night the Prophet stood for the prayer and the people followed him. This went on for two or three nights. Thereupon Allah's Apostle did not stand for the prayer the following night, and did not come out. In the morning, the people asked him about it. He replied, that he way afraid that the night prayer might become compulsory.