Sahih Muslim Book - 6, Hadith - 1748

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 خَشِىَ أَحَدُكُمُ الصُّبْحَ صَلَّى رَكْعَةً وَاحِدَةً تُوتِرُ لَهُ مَا قَدْ صَلَّى ».

It was narrated from Ibn 'Umar that a man asked the Messenger of Allah (s.a.w) about prayers at night. The Messenger of Allah (s.a.w) said: "The night prayers are two by two, then if one of you fears that dawn may come, he should pray one Rak'ah, which will make what he has prayed odd-numbered."