Sahih Muslim Book 004, Hadith Number 1712.
Chapter : Preference for observing Nafl prayer in the house and permissibility of observing it in the mosque also.
Alqama reported: I asked 'Aisha, the mother of the believers, saying O mother of the believers, how did the Messenger of Allah (may peace be upon him) act? Did he choose a particular act for a particular day? She said: No. He act was continuous, and who amongst you is capable of doing what the Messenger of Allah (may peace be upon him) did?