Sunan an-Nasa'i 1671
It was narrated from Ibn Umar that:
The Prophet (ﷺ) 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 ‏"‏ ‏.‏