Hadith: The Witr is due, so whoever wants to pray the Witr as seven (units of prayer), let him do so; whoever wants to pray the Witr as five, let him do so; whoever wants to pray the Witr as three, let him do so; and whoever wants to pray the Witr as one, let him do so

عن أبي أيوب الأنصاري رضي الله عنه أن النبي صلى الله عليه وسلم قال: «الوِتر حَق، فمن شاء أوْتَر بِسبْعٍ، ومن شاء أوْتَر بخمس، ومن شاء أوْتَر بثلاث، ومن شاء أوْتَر بواحدة».
[صحيح] - [رواه النسائي وأبو داود وابن ماجه]

Abu Ayyūb al-Ansāri (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "The Witr is due, so whoever wants to pray the Witr as seven (units of prayer), let him do so; whoever wants to pray the Witr as five, let him do so; whoever wants to pray the Witr as three, let him do so; and whoever wants to pray the Witr as one, let him do so."
Sahih/Authentic. - [Ibn Maajah]

Explanation

Meaning of the Hadīth: "The Witr is due" means that the Witr is either an affirmed act of the Sunnah or that it is an obligation. In this Hadīth, however, the Witr is affirmed, for there are explicit proofs that it is not an obligation, such as the following: - Talhah ibn ‘Ubaydullāh, a man from Najd, asked the Prophet (may Allah's peace and blessings be upon him) about the prayers to observe; he was told to observe only five prayers in the day and the night. Other prayers would be voluntary [Al-Bukhāri and Muslim]. Since the Witr was not included, it is not obligatory. - The Prophet (may Allah's peace and blessings be upon him) said : "Five prayers that Allah enjoined on His slaves. For anyone who performs them and misses nothing from them out of neglect, there is a pledge from Allah to admit him into Paradise.'' Proofs that Witr is not obligatory: - Ibn ‘Abbās (may Allah be pleased with him) reported that the Prophet sent Mu‘ādh to Yemen to tell them, among other things, that Allah enjoined five prayers on them to perform in the day and the night [Al-Bukhāri and Muslim]. That is evidence is strong because the Prophet sent Mu‘ādh to Yemen just before the Prophet died. - ‘Ali (may Allah be pleased with him) said: "The Witr is not obligatory." Therefore, being ''due'', the Witr is a confirmed act of the Sunnah. It could be prayed as seven, five, three, or one unit of prayer. In principle, optional night prayer is made two by two Rak‘ahs, as a Hadīth indicates [Al-Bukhāri and Muslim]. However, it is possible to pray all night units of prayer with one Tashahhud. Umm Salamah (may Allah be pleased with her) reported that the Prophet used to perform the Witr as seven or five Rak‘ahs consecutively, without ending them with the Salām greeting or speaking in between. [Musnad Ahmad] ‘Ā'ishah reported that the Prophet used to perform the Witr as five Rak‘ahs and sit only in the last one. [Abu Dawūd] As for the Witr which consists of three units of prayer, Ubayy ibn Ka‘b reported that the Messenger of Allah (may Allah's peace and blessings be upon him) used to recite Sūrat al-A‘lā in [the first Rak‘ah] the Witr, Sūrat al-Kāfirūn in the second, and Sūrat al-Ikhlās in the third. He would not say the Salām greeting except once after the third unit of prayer. [An-Nasā'i] Ibn ‘Uthaymīn (may Allah have mercy upon him) said: It is permissible to pray the Witr as three, five, seven or nine Rak‘ahs. If one prays three, he has two options: The first one is to pray three Rak‘ahs consecutively with one Tashahhud; the second one is to say the Salām greeting after the first two Rak‘ahs and then offer the last one. It is preferable to say the Salām greeting after every two Rak‘ahs and then finish the Witr with one Rak‘ah as in doing so, one does extra worship, and it is in agreement with the Prophet's practice.