(5) باب مَا جَاءَ فِي الْوِتْرِ بِسَبْعٍ
Umm Salamah narrated:
"The Prophet would perform Witr with thirteen [Rak'ah]. When he was older and became weak he performed Witr with seven." (ID)
Reference (ID): Jami` at-Tirmidhi 457, 458 (ID)
In-book reference (ID): Book 3, Hadith 6 (ID)
English translation (ID): Vol. 1, Book 3, Hadith 457 (ID)