Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.013.
It is permissible to perform the witr by praying two rak’at [and concluding them] and then praying one rak’ah with a tashahud and taslim. Likewise, it is allowed to pray all the rak’at with two tashahuds and one taslim. One may pray a number of rak’at, one after another, without making any tashahud, save in the one before the last rak’ah in which case one makes the tashahud and then stands to perform the last rak’ah wherein one will make another tashahud and end the prayer with the taslim. One may also make only one tashahud and the taslim, in the last rak’ah of witr. All of that is permissible and can be traced to the Prophet.
Talking about the thirteen rak’at in witr, at-Tirmizhi says: “It has been related from the Prophet that he would perform the witr prayer with thirteen, nine, seven, five, three rak’at or one rak’ah.”
On the other hand, Ishaq ibn Ibrahim holds: “The meaning of the statement that the Prophet prayed thirteen rak’at of witr is that during the night he would pray thirteen rak’at including the witr prayer, and so all of the night prayer came to be known as witr.”
Ibn al-Qayyim’s view is that “the clear, authentic sunnah is to pray the witr with five or seven connected rak’at as reported by Umm Salamah in her hadith. [She says] that the Prophet would perform the witr with five or seven rak’at without breaking them apart with taslim or any speech.” This is related by Ahmad, an-Nasa’i, and Ibn Majah with a good chain.
As previously mentioned, al-Bukhari and Muslim quote ‘Aishah saying that the Prophet would perform thirteen rak’at during the night and would make the witr prayer with five of them, and he would not ‘sit’ [during those five] except in the last rak’ah of them. In another hadith, ‘Aishah reports that the Prophet sallallahu alehi wasallam would perform nine rak’at during the night and that he would not sit during them until the eighth rak’ah in which he would make remembrance of Allah, praising Him, and would make supplication. Then, he would stand without making the taslim and pray the ninth rak’ah, after which he would sit, make the tashahud and make the taslim in such a manner that we could hear him. Then, he would pray two rak’at after the taslim while sitting, and that would make eleven rak’at. When he became older and heavier, he would make the witr with seven rak’at, performing the (last) two rak’at like the first one. In another version from her, it is stated: “When he became older and bulkier, he would make the witr with seven rak’at, and he would not sit during them, save in the sixth and seventh rak’ah and he would not make the taslim, save in the seventh rak’ah.” In yet another version, it is stated: “He would pray seven rak’at and would not sit, save in the last of them.” This is related by the group.
All of the preceding ahadith are authentic and clear and there is no contradiction in them. As to the Prophet’s statement: “The night prayer is in sets of two [rak’at],” it is not relevant here. This is an authentic hadith, and the statement that he observed witr with seven or five rak’at is equally true. Both statements confirm each other. The seven, five, nine, and one rak’ah constitute the witr prayer, for witr is the name given to the one rak’ah offered in conclusion of whatever is offered prior to it. And the witr of the five, seven and nine rak’at are all connected like the maghrib which is described as three connected rak’at. If one breaks apart the five or seven rak’at with two taslim, like in the eleven rak’at, it will all be called witr due to the last odd rak’ah. This is supported by the Prophet’s statement: ‘The night prayer is sets of two rak’at. If one fears the coming of the dawn, he should perform one rak’ah, thereby making all of them odd [witr].’ Therefore, the Prophet’s actions and statements are in agreement, each part confirming the other.” The fact is that the Prophet was responding to a question about the night prayer when he said: “it is in pairs of two.” He was not speaking about witr, for the man had asked him about night prayer, and not about the witr.