LENGTH OF THE VOLUNTARY PRE-DAWN PRAYER H 1127 "The Messenger of Allah, praise and peace be upon him, offered his voluntary pairs of two units of prayer at night (up to 8) then two then concluded with one unit of prayer thereby making the number odd (witr). He would offer two units of prayer before the dawn prayer so quickly that the second call to prayer coincided with the length of his prayer." R 1127 Bukhari and Muslim with a chain up to ibn Omar who related this.
1106- وعن ابن عمر رَضِيَ اللهُ عَنهُما ، قَالَ : كَانَ رسول الله صلى الله عليه وآله وسلم يُصَلِّي مِنَ اللَّيْلِ مَثْنَى مَثْنَى ، وَيُوتِرُ بِرَكْعَةٍ مِنْ آخِرِ اللَّيْلِ ، وَيُصَلِّي الرَّكْعَتَيْنِ قَبْلَ صلاةِ الغَدَاةِ ، وَكَأنَّ الأَذَانَ بِأُذُنَيْهِ . متفقٌ عَلَيهِ .