Mishkat al-Masabih 4 Prayer كتاب الصلاة 697

وَعَنْ ثَوْبَانَ عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «إِنَّ هَذَا السَّهَرَ جُهْدٌ وَثِقَلٌ فَإِذَا أَوْتَرَ أَحَدُكُمْ فَلْيَرْكَعْ رَكْعَتَيْنِ فَإِنْ قَامَ مِنَ اللَّيْلِ وَإِلَّا كَانَتَا لَهُ» . رَوَاهُ الدَّارِمِيُّ

Thauban reported the Prophet as saying, “This wakefulness is a toil and a burden, so when one of you observes a witr he should pray two rak’as . If he gets up during the night, well and good; otherwise they will serve for him.” Tirmidhi transmitted it.