Mishkat al-Masabih 4 Prayer كتاب الصلاة 669

عَنِ ابْنِ عُمَرَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «صَلَاةُ اللَّيْلِ مَثْنَى مَثْنَى فَإِذَا خَشِيَ أَحَدُكُمُ الصُّبْحَ صَلَّى رَكْعَةً وَاحِدَة توتر لَهُ مَا قد صلى»

Ibn ‘Umar reported God’s Messenger as saying, “Prayer during the night should consist of pairs of rak’as , but if one of you fears the morning is near he should pray one rak’a which will make his prayer an odd number for him.” (Bukhari and Muslim.)