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 ". أَنَّ رَجُلاً نَادَى النَّبِيَّ صلى الله عليه وسلم وَهُوَ فِي الْمَسْجِدِ.
Ibn Umar reported that a man came to the Prophet ﷺ while he was delivering a sermon and said, “How is the night prayer?” He said, “Two by two. When you fear (the coming of) dawn, pray witr with one (rak'ah); it makes what you have prayed …
also in Sahih al-Bukhari 472