Sunan an-Nasa'i, Hadith 1671 ⓘ
It was narrated from Ibn Umar that: The Prophet said: "prayers at night are (offered) two by two, then if you fear that dawn will come, pray witr with one."
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى فَإِذَا خِفْتَ الصُّبْحَ فَأَوْتِرْ بِوَاحِدَةٍ " .
also referenced under: Book 20, Hadith 74 (In-Book) & Volume 2, Book 20, Hadith 1672 (DarusSalam)