Hadith: While the Prophet (may Allah's peace and blessings be upon him) was standing on the pulpit, a man asked him: What do you say about prayer in the night? He replied: Two and two; and if one is afraid that the Fajr is coming, then one should offer one Rak‘ah, which will be the Witr for whatever one has prayed

عن ابن عمر رضي الله عنهما قال: سَأل رَجُل النبي -صلَّى الله عليه وسلم- وهو على المِنْبَر، ما تَرى في صلاة الليل؟ قال: « مَثْنَى مَثْنَى، فإذا خَشْيَ الصُّبح صلَّى واحِدَة، فأَوْتَرت له ما صلَّى» وإنَّه كان يقول: اجْعَلُوا آخِر صَلاَتِكُمْ وتْرَا، فإنَّ النبِيَّ -صلَّى الله عليه وسلَّم- أمَر بِه. وفي رواية: فقيل لابن عمر: ما مَثْنَى مَثْنَى؟ قال: «أن تُسَلِّم في كل ركعتين».
[صحيح] - [متفق عليه]

Ibn ‘Umar (may Allah be pleased with him) reported: While the Prophet (may Allah's peace and blessings be upon him) was standing on the pulpit, a man asked him: "What do you say about prayer in the night?" He replied: "Two and two; and if one is afraid that the Fajr is coming, then one should offer one Rak‘ah, which will be the Witr for whatever one has prayed." Ibn ‘Umar (may Allah be pleased with him) used to say: "Make the Witr the last of your prayer. This was commanded by the Prophet (may Allah’s peace and blessings be upon him)." In another narration: Someone asked Ibn ‘Umar: "What is two and two?" He replied: "To make Taslīm after every two Rak‘ahs."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

"While the Prophet (may Allah's peace and blessings be upon him) was standing on the pulpit, a man asked him: 'What do you say about prayer in the night?'" In other words: What is the Shar‘i ruling which your Lord taught you about the number of Rak‘ahs of this prayer and whether to connect them or separate them by Taslīm? In another narration by Al-Bukhāri and Muslim: "How to offer the prayer at night?" "He replied: 'Two and two.'" The word is repeated for more emphasis. The Prophet’s reply indicates that a Muslim should make Taslīm after every two Rak‘ahs in the prayer at night, as explained by Ibn ‘Umar (may Allah be pleased with him). The Witr is an exception to this, however. If he offers the Witr as three, five, or seven Rak‘ahs, he may pray all of them together and then make Taslīm in the last Rak‘ah. "And if one is afraid that the Fajr is coming, one should offer one Rak‘ah." That is to say that if a person fears that the Fajr is about to come, he should hasten to offer one Rak‘ah, with Tashahhud and Taslīm. "Which will be the Witr for whatever one has prayed." This is because when one Rak‘ah is added to the even number of Rak‘ahs, the whole prayer becomes odd in number. Ibn ‘Umar (may Allah be pleased with him) used to say: "Make the Witr the last of your prayer. This was commanded by the Prophet (may Allah’s peace and blessings be upon him)." A subnarrator of the Hadīth called Nafi‘ informed that Ibn ‘Umar (may Allah be pleased with him) used to make this statement. In another narration by Muslim: "Make the Witr the last of your prayer at night." In other words, offer the last of your Tahajjud (night prayer) in the night as the Witr. Then Ibn ‘Umar (may Allah be pleased with him) pointed out that this statement is not his personal opinion but a Prophetic instruction: "This was commanded by the Prophet (may Allah’s peace and blessings be upon him)." As the Maghrib prayer is considered the Witr for the prayers offered during the day, the Witr prayer is the same for the prayer at night. In another narration: "Someone asked Ibn ‘Umar: 'What is two and two?'" That is to say: What does two and two mean? Clarifying the meaning intended by the Prophet (may Allah's peace and blessings be upon him) Ibn ‘Umar (may Allah be pleased with him) replied: "To make Taslīm after every two Rak‘ahs." In other words: Offer two Rak‘ahs and make Taslīm, and then two Rak‘ahs and Taslīm, and so on, adding nothing to the two-Rak‘ah unit.