He reported God’s Messenger as saying, “The <i>witr</i>* is a <i>rak'a</i> at the end of the night.” * Literally ‘single’, or ‘odd’, used of an odd number of <i>rak'as</i> prayed at some time during the night. Muslim transmitted it.

وَعَنِ ابْنِ عُمَرَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «الْوَتْرُ رَكْعَةٌ مِنْ آخر اللَّيْل» . رَوَاهُ مُسلم