Sahih Bukhari Volume 001, Book 011, Hadith Number 696.
Narated 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.