Umm al-Mu’minin Umm Salamah (may Allah be pleased with her) says: The Prophet (peace and blessings of Allah be upon him) used to pray thirteen rak‘ahs of Witr, but when he became old and weak, he started praying seven rak‘ahs. Imam Tirmidhi says:
1- The hadith of Umm Salamah (may Allah be pleased with her) is Hasan (good).
2- In this chapter, there is also a narration from ‘A’ishah (may Allah be pleased with her).
3- From the Prophet (peace and blessings of Allah be upon him), Witr with thirteen, eleven, nine, seven, five, three, and one (rak‘ah) are all narrated.
4- Ishaq bin Ibrahim (Ibn Rahwayh) says: This narration that the Prophet (peace and blessings of Allah be upon him) used to pray thirteen rak‘ahs of Witr means that he used to pray Tahajjud (Qiyam al-Layl) along with Witr, making thirteen rak‘ahs, so in this, Qiyam al-Layl (Tahajjud) is also called Witr. He has also narrated a hadith from ‘A’ishah (may Allah be pleased with her) in this regard, and has taken evidence from the statement of the Prophet (peace and blessings of Allah be upon him):“O people of the Qur’an! Pray Witr.”Ibn Rahwayh says that by this, the Prophet (peace and blessings of Allah be upon him) meant Qiyam al-Layl, and Qiyam al-Layl is only for those who believe in the Qur’an (not for followers of other religions).

Ishaq bin Ibrahim (Ibn Rahwayh) says: The narration that the Prophet (peace be upon him) used to pray thirteen rak‘ahs of witr means that he used to pray thirteen rak‘ahs of tahajjud (qiyam al-layl) including witr, so in this, qiyam al-layl (tahajjud) has also been called witr. He has narrated a hadith from Aisha (may Allah be pleased with her) in this regard, and has also taken evidence from the saying of the Prophet (peace be upon him): "O people of the Qur’an! Pray witr." Ibn Rahwayh says that by this, the Prophet (peace be upon him) meant qiyam al-layl, and qiyam al-layl is only obligatory upon those who believe in the Qur’an (not upon followers of other religions).