وَعَنِ ابْنِ عُمَرَ قَالَ: قَالَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-{ مَنْ أَدْرَكَ رَكْعَةً مِنْ صَلَاةِ اَلْجُمُعَةِ وَغَيْرِهَا فَلْيُضِفْ إِلَيْهَا أُخْرَى, وَقَدْ تَمَّتْ صَلَاتُهُ } رَوَاهُ النَّسَائِيُّ, وَابْنُ مَاجَهْ, وَاَلدَّارَقُطْنِيُّ, وَاللَّفْظُ لَهُ, وَإِسْنَادُهُ صَحِيحٌ, لَكِنْ قَوَّى أَبُو حَاتِمٍ إِرْسَالَهُ.‏ 1‏.‏

‏1 ‏- صحيح.‏ رواه النسائي (1/274‏-275)‏، وابن ماجه (1123)‏، والدارقطني (2/12/12)‏.‏

The Witr prayer is not obligatory as the prescribed prayer is [i.e. the five daily prayers], but it is a Sunnah which was established by the practice of Allah's Messenger (ﷺ).[at-Tirmidhi and an-Nasa'i reported it, who graded it Hasan (good). al-Hakim graded it Sahih (authentic)].