Sahih Muslim Book 4, Hadith Number 1614.

Chapter : Permissibility of observing Nafl (voluntary prayer) standing or sitting and observing some part of it standing or sitting.

Masruq is reported to have asked ‘Aisha about the action (most pleasing to) the Messenger of Allah (may peace be upon him). She said: He (the Holy Prophet) loved (that action) which one keeps on doing regularly. I said (to ‘Aisha): When did he pray (at night)? She replied: When he heard the cock crow, he got up and observed prayer.