The Prophet ﷺ said, "Pray the Witr before morning comes."
أن النبي ﷺ قال: أوتروا قبل أن تصبحوا
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Muslim narrated it in the Book of the Prayer of Travelers (754), from Abu Bakr bin Abu Shayba, who said, Abdul-Ala bin Abdul-Ala narrated to us, from Mamar, from Yahya bin Abu Kathir, from Abu Nadra, from Abu Saeed, and he mentioned it. Then Muslim narrated it by another route from Yahya bin Abu Kathir, who said, Abu Nadra Awqi, who is Munzir bin Malik bin Qita, informed me that Abu Saeed informed them that they asked the Prophet ﷺ about Witr, and he mentioned it.Imam Ahmad narrated it (11001) by another route from Yahya bin Abu Kathir, from Abu Nadra, from Abu Saeed, that the Prophet ﷺ said, "Witr is at night," and its chain is sahih. This is a summarized rendering of the meaning of his statement, "Perform Witr before you enter the morning."Ibn Majah narrated this hadith (1189) from Muhammad bin Yahya, from Abdul-Razzaq, from Mamar, from Yahya bin Abu Kathir, with the same chain. Muhammad bin Yahya said, "In this hadith there is evidence that the hadith of Abdul-Rahman is weak."I say, he is pointing to the hadith that comes after it in the chapter on making up missed Witr.