The Prophet ﷺ said, "Make the last of your night prayers an odd-numbered one."
عن النبي ﷺ قال: اجعلوا آخر صلاتكم بالليل وترا.
Bukhari related it in the book of Witr (998) and Muslim in the book of prayer of the traveler (751/151), both from the hadith of Yahya bin Saeed, from Ubaydullah, from Nafi, from Ibn Umar, as mentioned.Muslim also related it through Ibn Jurayj, who said, Nafi informed me that Ibn Umar used to say, "Whoever prays at night should make the last of his prayer Witr before dawn." That is how the Messenger of Allah ﷺ used to command them. End of quote.Nothing has been established as marfu regarding the nullification of Witr. Rather, that has been related from Abdullah bin Umar, meaning that if he slept having already prayed Witr, then got up to pray during the night, he would pray one rakat added to his previous Witr, thereby making it an even number, and then he would pray Witr again at the end of his prayer, as Malik related in Muwatta, in the book of night prayer (19), from Nafi, who said, I was with Abdullah bin Umar in Makkah, and the sky was overcast. Abdullah feared that dawn would come, so he prayed Witr with a single rakat. Then the clouds cleared, and he saw that there was still some night left, so he made it even by adding one rakat, then he prayed after that two rakats at a time. Then, when he feared dawn was near, he prayed Witr with a single rakat.A statement similar to that of Ibn Umar has also been related from Ali bin Abu Talib, Uthman, Abdullah bin Masud, and Usamah, and this is the view held by a number of scholars, among them Urwa bin Zubayr, Makhul, Amr bin Maymun, and others. Their argument for this is the hadith of Ibn Umar, "Make the last of your prayer at night Witr," and Bukhari titled his chapter accordingly, as though he leaned toward this view. Another argument of theirs is his saying, ﷺ, "Witr is one rakat at the end of the night," and his saying, ﷺ, "So when any of you fears the approach of dawn, let him pray Witr with a single rakat."As for the prohibition against praying two Witrs in one night, that has been related from Qays bin Talq, from his father, as will appear in the following chapter.