Sahih Muslim 741
Sahih Muslim · Book 6: The Book of Prayer - Travellers · Night prayers and the number of rak`ah offered by the Prophet (saws) at night, and that Witr is one rak`ah, and a one-rak`ah prayer is correct
Masruq is reported to have asked 'A'isha about the action (most pleasing to) the Messenger of Allah (ﷺ). She said:
حَدَّثَنِي هَنَّادُ بْنُ السَّرِيِّ ، حَدَّثَنَا أَبُو الأَحْوَصِ ، عَنْ أَشْعَثَ ، عَنْ أَبِيهِ ، عَنْ مَسْرُوقٍ ، قَالَ سَأَلْتُ عَائِشَةَ عَنْ عَمَلِ، رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَتْ كَانَ يُحِبُّ الدَّائِمَ . قَالَ قُلْتُ أَىَّ حِينٍ كَانَ يُصَلِّي فَقَالَتْ كَانَ إِذَا سَمِعَ الصَّارِخَ قَامَ فَصَلَّى .
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.