Sahih al-Bukhari, Book 8, Hadith 121 (In-Book) ⓘ

Narrated Ibn `Umar: A man came to the Prophet while he was delivering the sermon and asked him how to offer the night prayers. The Prophet replied, 'Pray two rak`at at a time and then two and then two and so on and if you are afraid of dawn (the approach of the time of the Fajr prayer) pray one rak`a and that will be the witr for all the rak`at which you have prayed." Narrated 'Ubaidullah bin `Abdullah bin `Umar: A man called the Prophet while he was in the mosque.

حَدَّثَنَا أَبُو النُّعْمَانِ، قَالَ حَدَّثَنَا حَمَّادٌ، عَنْ أَيُّوبَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَجُلاً، جَاءَ إِلَى النَّبِيِّ صلى الله عليه وسلم وَهْوَ يَخْطُبُ فَقَالَ كَيْفَ صَلاَةُ اللَّيْلِ فَقَالَ ‏ "‏ مَثْنَى مَثْنَى، فَإِذَا خَشِيتَ الصُّبْحَ فَأَوْتِرْ بِوَاحِدَةٍ، تُوتِرُ لَكَ مَا قَدْ صَلَّيْتَ ‏" ‏‏.‏ قَالَ الْوَلِيدُ بْنُ كَثِيرٍ حَدَّثَنِي عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ أَنَّ ابْنَ عُمَرَ حَدَّثَهُمْ أَنَّ رَجُلاً نَادَى النَّبِيَّ صلى الله عليه وسلم وَهُوَ فِي الْمَسْجِدِ‏.‏