Sahih Muslim Book 4, Hadith Number 1666.

Chapter : Exhortation to supplicate and make mention (of Allah) at the end of the night and its acceptance by the Lord.

‘Aisha reported that the Messenger of Allah (may peace be upon him) prayed one night in the mosque and people also prayed along with him. He then prayed on the following night and there were many persons. Then on the third or fourth night (many people) gathered there, but the Messenger of Allah (may peace be upon him) did not come out to them (for leading the Tarawih prayer). When it was morning he said: I saw what you were doing, but I desisted to come to you (and lead the prayer) for I feared that this prayer might become obligatory for you. (He the narrator) said: It was the month of Ramadan.