VOLUNTARY NIGHT PRAYER H 1191 "The night prayer consists of two units of prayer offered one after each other. When you expect the approach of dawn complete them with a single unit of prayer to make them an odd number." R 1191 Bukhari and Muslim with a chain up to ibn Omar who related that the Messenger of Allah, praise and peace be upon him, said this.
1168- وعن ابن عمر رضي الله عنهما : أنَّ النبيَّ صلى الله عليه وآله وسلم ، قَالَ : "صَلاةُ اللَّيْلِ مَثْنَى مَثْنَى ، فَإذَا خِفْتَ الصُّبْحَ فَأوْتِرْ بِوَاحِدَةٍ" متفقٌ عَلَيْهِ .