كتاب صلاة المسافرين وقصرها
6  
The Book of Prayer - Travellers
(20)
Chapter: The night prayers are two by two, and Witr is one rakah at the end of the night
(20)
باب صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى وَالْوِتْرُ رَكْعَةٌ مِنْ آخِرِ اللَّيْلِ ‏
Sahih Muslim 749 h

Anas b. Sirin reported:

I asked Ibn 'Umar like this (as recorded in the previous hadith) and he made this addition:" And he (the Holy Prophet) made the end of the night prayer as odd number by one rak'ah." And there is also (this addition):" Stop, stop, you are bulky."
وَحَدَّثَنَا ابْنُ الْمُثَنَّى، وَابْنُ، بَشَّارٍ قَالاَ حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ أَنَسِ بْنِ سِيرِينَ، قَالَ سَأَلْتُ ابْنَ عُمَرَ ‏.‏ بِمِثْلِهِ وَزَادَ وَيُوتِرُ بِرَكْعَةٍ مِنْ آخِرِ اللَّيْلِ ‏.‏ وَفِيهِ فَقَالَ بَهْ بَهْ إِنَّكَ لَضَخْمٌ ‏.‏
Reference : Sahih Muslim 749h
In-book reference : Book 6, Hadith 191
USC-MSA web (English) reference : Book 4, Hadith 1646
  (deprecated numbering scheme)