Sahih Bukhari Volume 002, Book 013, Hadith Number 046.
Narated By 'Aisha : Once in the middle of the night Allah's Apostle (p.b.u.h) went out and prayed in the mosque and some men prayed with him. The next morning the people spoke about it and so more people gathered and prayed with him (in the second night). They circulated the news in the morning, and so, on the third night the number of people increased greatly. Allah's Apostle (p.b.u.h) came out and they prayed behind him. On the fourth night the mosque was overwhelmed by the people till it could not accommodate them. Allah's Apostle came out only for the Fajr prayer and when he finished the prayer, he faced the people and recited "Tashah-hud" (I testify that none has the right to be worshipped but Allah and that Muhammad is His Apostle), and then said, "Amma ba'du. Verily your presence (in the mosque at night) was not hidden from me, but I was afraid that this prayer (Prayer of Tahajjud) might be made compulsory and you might not be able to carry it out."