The Book of Prayer - Travellers
6/20. Chapter:
The night prayers are two by two, and Witr is one rakah at the end of the night
Sahih Muslim
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ نَافِعٍ، وَعَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَجُلاً، سَأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم عَنْ صَلاَةِ اللَّيْلِ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى فَإِذَا خَشِيَ أَحَدُكُمُ الصُّبْحَ صَلَّى رَكْعَةً وَاحِدَةً تُوتِرُ لَهُ مَا قَدْ صَلَّى " .
Ibn 'Umar reported that a person asked the Messenger of Allah (ﷺ) about the night prayer. The Messenger of Allah (ﷺ) said: Prayer during the night should consist of pairs of rak'ahs, but if one of you fears morning is near, he should pray one rak'ah which will make his prayer an odd number for him.
Reference : Sahih Muslim 749aIn-book reference : Book 6, Hadith 177USC-MSA web (English) reference : Book 4, Hadith 1632