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 ‏.‏

Ibn 'Umar reported Allah's Messenger (PBUH) as saying: 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. Translation Not Available

Book Ref: Sahih Muslim Book 6 Hadith 1647
Web Ref:  Sahih Muslim Book 4 Hadith 749 i