Masruq is reported to have asked 'A'isha 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 'A'isha): When did he pray (at night)? She replied: When he heard the cock crow, he got up and observed prayer.
[Hadith from: Sahih Muslim, Book No. 4: 'The Book of Prayers (Kitab Al-Salat)', Hadith No. 1614]