Sahih Bukhari Volume 002, Book 021, Hadith Number 229.

Narated By 'Aisha : (The mother of the faithful believers) One night Allah's Apostle offered the prayer in the Mosque and the people followed him. The next night he also offered the prayer and too many people gathered. On the third and the fourth nights more people gathered, but Allah's Apostle did not come out to them. In the morning he said, "I saw what you were doing and nothing but the fear that it (i.e. the prayer) might be enjoined on you, stopped me from coming to you." And that happened in the month of Ramadan.