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.
أَنَّ رَسُولَ اللَّهِ ﷺ 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. (Using translation from Muslim 749i)
عَنْ رَسُولِ اللهِ ﷺ قَالَ صَلَاةُ اللَّيْلِ مَثْنَى مَثْنَى فَإِذَا رَأَيْتَ أَنَّ الصُّبْحَ يُدْرِكُكَ فَأَوْتِرْ بِوَاحِدَةٍ قَالَ فَقِيلَ لِابْنِ عُمَرَ مَا مَثْنَى مَثْنَى قَالَ تُسَلِّمُ فِي كُلِّ رَكْعَتَيْنِ
[Machine] The Messenger of Allah ﷺ said, "The night prayer should be performed in sets of two. When you see that dawn is approaching, then conclude with one unit of prayer." A man asked Ibn Umar, "What does 'sets of two' mean?" He replied, "You make Salaam after every two units of prayer."
قَالَ رَسُولُ اللهِ ﷺ صَلَاةُ اللَّيْلِ مَثْنَى مَثْنَى فَإِذَا رَأَيْتَ أَنَّ الصُّبْحَ مُدْرِكُكَ فَأَوْتِرْ بِرَكْعَةٍ فَقَالَ رَجُلٌ لِابْنِ عُمَرَ مَا مَثْنَى؟ قَالَ تُسَلِّمُ فِي كُلِّ رَكْعَتَيْنِ
"عَنْ عُقْبَةَ بْنِ حُرَيْث قَالَ: سَمِعْتُ ابْنَ عُمَرَ يُحَدِّثُ أن رَسُولَ الله ﷺ قَالَ: صَلاَةُ اللَّيْلِ مَثْنَى مَثْنَى، فَإذَا رأَيْتَ الصُّبْحَ يُدْرِكُكَ فَأوْتِرْ بِوَاحِدَةٍ، فَقِيلَ لابنِ عُمَرَ: مَا مَثْنَى مَثْنَى؟ قالَ: تُسَلِّمُ فِى كُلِّ رَكْعَتَيْنِ".