Talhah ibn 'Ubaydillah said, "A man from Najd with unkempt hair came to the Messenger of Allah, and we heard his loud voice but could not understand what he was saying, until he came near and then we came to know that he was asking about Islam. The Messenger of Allah said, '(You have to offer) five prayers each day and night.' The man asked, 'Do I have to do anything else?' The Messenger of Allah said, 'No, unless you want to offer voluntary (nafl) prayers.'" [Bukhari and Muslim]
There are longer versions of this hadith, which includes mention of the other pillars of Islam, but since I'm writing about salah what is written above will suffice inshaaAllaah.
The man from Najd in this case is a bedouin, and though the Prophet ﷺ and his Companions lived in Arabia they were not all bedouins. Mostly they considered themselves to be city people, whereas the man from Najd was what we might think of as a country person. So we can think today of someone living out in the country--that they might tend to be in their manners and speech less refined, and less subtle.
In this hadith, the man is basically asking for the minimum he has to do in Islam--the very basic question as to what is obligatory. And the man said that he would do this much, the five obligatory prayers, and not add anything to them of what was nafl or voluntary. And the Prophet ﷺ told his Companions after this incident that if the man kept his word, and kept up the five obligatory prayers, then he'd be good to go.
So what are the five obligatory prayers? The bare minimum we have to do as Muslims, and without which we will be deficient? They are fajr, dhuhr, 'asr, maghrib, and 'isha'.
Fajr prayer is the dawn prayer, consisting of two rak'ahs. Dhuhr is prayed after noon and is four rak'ahs. 'Asr is prayed in the late afternoon, also four rak'ahs, followed by maghrib immediatley after sunset which is only three rak'ahs, and the day concludes with 'isha', the night prayer, also consisting of four rak'ahs.
The witr salah is not included among these five, but the ruling on it basically includes two opinions, that it is a recommended sunnah (according to Imams Malik, Ash-Shafi'i, and Ahmad), and the second opinion that it is waajib, i.e., obligatory, (the opinion of Abu Hanifa). The question has come up even in some reports of hadith, and there are evidences to both sides. The Hanafi school would consider an obligation waajib if it were based on a hadith that was not mutawaatir (multiply narrated,) whereas Qur'an and mutawaatir hadith were sources of obligations classified as 'fard.' For the other schools, the terms waajib and fard are mostly interchangeable, and a hadith could be used to classify something as fard without being mutawaatir, provided that it was authentic.
The evidence to support the Hanafi opinion are the following two ahadith:
'Abdullah ibn 'Amr ibn al-'As narrated that the Messenger said, "Verily, Allah has added on you a salah, which is the witr." [Ahmad]The evidence to support the opinion of Imams Malik, Ash-Shafi'i, and Ahmad, that witr is a recommended (mu'akkadah) sunnah are the following two ahadith:
Abu Ayyub narrated that the Messenger said, "Witr is haqq (obligation) on every Muslim."
'Ali ibn Abu Talib reported, "The witr prayer is not obligatory as the prescribed salah, but the Messenger of Allah observed it as his regular practice (sunnah.) He (the Prophet) said, 'Allah is witr (single) and loves what is witr. So perform witr salah. O followers of the Qur'an, observe witr salah.'" [At-Tirmidhi and Abu Dawud]I mention both opinions and their evidence lest anyone wish to dispute the matter. What is clear is that we have as an obligation (at minimum) five daily prayers. Allah calls us to remember him (at minimum) five times a day, five daily opportunities for spiritual nourishment. And for five times during the day we have a means of forgiveness for sins committed throughout the day and night. As Muslims we should build our lives around the salah, let it organize our day and serve as a tool for self-discipline.
Al-Mukhdaji heard from one of the ansar, nicknamed Abu Muhammad, that the witr prayer is obligatory. He went to 'Ubadah ibn us-Samit, and mentioned to him what Abu Muhammad had said. 'Ubadah observed, "Abu Muhammad is mistaken, for I heard the Messenger of Allah say, 'Five prayers are ordained by Allah for his slaves. Whoever fulfills them properly without any shortcoming, he will have a pact with Allaah that He will admit him into Paradise. Whoever does not do them, he will have no pact with Allah, and if He wills He may punish him and if He will He may forgive him.'" [Ahmad and Abu Dawud]
Five prayers a day is a blessing as much as an obligation, a mercy and means of purification.
Abu Hurayrah narrated that the Messenger of Allah said, "If there was a river at the door of any one of you and he took a bath in it five times a day, would you notice any dirt on him?" They said, "Not a trace of dirt would be left." The Prophet added, "That is the example of the five prayers with which Allah blots out evil deeds." [Bukhari and Muslim]
So when we next approach our prayer, we can be grateful for the opportunity to pray, to devote a small amount of our time each day to worshiping our Lord.