Nasai

1683. Commentary: From these narrations, it is understood that the Witr prayer can be performed after the ‘Isha prayer until the appearance of Fajr. However, whoever wishes to perform Tarawih and Tahajjud should offer Witr at the end of his voluntary (nafl) prayers, not at the beginning or in the middle. And Allah knows best.

Hadith Commentary:
(1)
The aforementioned prophetic command regarding making the last prayer of the night the witr is for recommendation (istihbab), because it is established from the Messenger of Allah (sallallahu alayhi wa sallam) that he would also perform two units (rak‘ahs) after the witr prayer. Thus, it is narrated from Umm Salamah (radi Allahu anha) that the Messenger of Allah (sallallahu alayhi wa sallam) used to perform two rak‘ahs after witr.
(Jami‘ at-Tirmidhi, Kitab as-Salat, Hadith: 471)
And the Messenger of Allah (sallallahu alayhi wa sallam) also encouraged the ummah to perform two rak‘ahs after witr, as is mentioned in the hadith in which he said:
“Waking up at dawn for the night prayer is very burdensome and difficult, so two rak‘ahs should be performed after witr; if one wakes up, that is better, otherwise these two will suffice for him.”
(Sahih Ibn Khuzaymah: 159/2)
On this hadith, the hadith scholar Ibn Khuzaymah (rahimahullah) has established a chapter with these words:
“It is permissible for one who wishes to perform supererogatory (nafl) prayer after witr to do so.”
The two supererogatory rak‘ahs that the Messenger of Allah (sallallahu alayhi wa sallam) used to perform after witr were not something specific to him, rather this practice is legislated for the entire ummah.
(Sahih Ibn Khuzaymah: 159/2)
(2)
Some individuals have tried to establish the obligation (wujub) of witr from this hadith, but this is not correct, because when the night prayer (salat al-layl) itself is not obligatory, then how can the prayer performed at its end be obligatory? Furthermore, the default ruling regarding any matter is non-obligation until its obligation is established by evidence.
(Fath al-Bari: 628/2)

1438. Commentary:
➊ Whoever is certain that he can wake up before dawn should act upon this instruction and attain the reward of virtue. Otherwise, the concession of performing witr before sleeping is known, as has been mentioned previously.
➋ It has been argued from this hadith that after performing witr, it is not permissible to offer any voluntary (nafl) prayer. However, other scholars have interpreted this as being preferable (mustahabb), because it is established from the Prophet (sallallahu alayhi wa sallam) himself that he performed two units (rak‘ah) of voluntary prayer after witr.

Takhrij:
«أخرجه البخاري، الوتر، باب ليجعل آخر صلاته وترًا، حديث:998، ومسلم، صلاة المسافرين، باب صلاة الليل مثني، حديث:751.»©Explanation:
➊ In this hadith, the command to make the last prayer of the night the witr is not for obligation, but rather for recommendation (istihbab). That is, if someone has already performed the witr in the early part of the night, and then wakes up in the middle or last part of the night, he may perform as many voluntary prayers (nawafil) as he wishes in sets of two rak‘ahs. There is no need to annul the witr by making it even.
➋ If a person performs two rak‘ahs after having performed the witr, there is no harm in it, because it is narrated in Sahih Muslim from ‘A’ishah radi Allahu anha that the Prophet sallallahu alayhi wa sallam used to perform two rak‘ahs sitting after the witr. (Sahih Muslim, Salat al-Musafirin, Hadith: 738 (b)).