The Prophet ﷺ said: "Prayers at night are two by two, then if you fear that dawn will come, pray witr with one rak'ah." (Using translation from Nasāʾī 1668)
أَنَّ النَّبِيَّ ﷺ قَالَ صَلَاةُ اللَّيْلِ مَثْنَى مَثْنَى فَإِذَا خَشِيتَ الْفَجْرَ فَأَوْتِرْ بِوَاحِدَةٍ