Sahih Muslim 749
Sahih Muslim · Book 6: The Book of Prayer - Travellers · The night prayers are two by two, and Witr is one rakah at the end of the night
Ibn 'Umar reported Allah's Messenger (ﷺ) as saying:
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى ، حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ ، حَدَّثَنَا شُعْبَةُ ، قَالَ سَمِعْتُ عُقْبَةَ بْنَ حُرَيْثٍ ، قَالَ سَمِعْتُ ابْنَ عُمَرَ ، يُحَدِّثُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى فَإِذَا رَأَيْتَ أَنَّ الصُّبْحَ يُدْرِكُكَ فَأَوْتِرْ بِوَاحِدَةٍ " . فَقِيلَ لاِبْنِ عُمَرَ مَا مَثْنَى مَثْنَى قَالَ أَنْ يُسَلِّمَ فِي كُلِّ رَكْعَتَيْنِ .
The night prayer consists of pairs and when you see the approach of dawn, make this number odd by one rak'ah. It was said to Ibn 'Umar: What does the (word) pair imply? He said: (It means) that salutation is uttered after every two rak'ahs.