"The Prophet would perform Witr with thirteen [Rak'ah]. When he was older and became weak he performed Witr with seven."  

كَانَ النَّبِيُّ ﷺ يُوتِرُ بِثَلاَثَ عَشْرَةَ رَكْعَةً فَلَمَّا كَبِرَ وَضَعُفَ أَوْتَرَ بِسَبْعٍ قَالَ وَفِي الْبَابِ عَنْ عَائِشَةَ