The scholars agree that it is obligatory for one who has forgotten the salah or slept through its time to make up the missed [qada’) prayer. This opinion is based on the hadith of the Prophet mentioned earlier: “There is no negligence while one is asleep but forgetfulness occurs when one is awake. If one of you forgets the prayer or sleeps through its time, then he should perform the salah when he recalls it.” If one falls unconscious, then he need not repeat the salah, unless he regains his consciousness with enough time to purify himself and perform the salah within its proper time.
‘Abdurrazaq relates from Naf’i that Ibn ‘Umar once fell sick and became unconscious and missed the prayer. When he regained his consciousness, he did not make up the missed prayer.
Ibn Juraij reports from Ibn Tawus on the authority of his father that if a sick person becomes unconscious, he is not to make up the prayers he missed .
Mu’ammar relates: “I asked az-Zuhri about one who becomes unconscious, and he said that he is not to make up the salah he missed.”
Hamad ibn Salamah relates from Yunus ibn ‘Ubaid that both al-Hassan al-Basri and Muhammad ibn Sireen said that a person who falls unconscious is not to make up the prayers he may miss.
Concerning missing a salah intentionally, the majority of the scholars say that it is a sin and the missed salah must be made up for. Ibn Taimaiyyah says:
He may however, resort to increasing his voluntary and supererogatory acts. Ibn Hazm has thoroughly discussed this question. The following is a summary of what he says on this subject:
Concerning one who leaves a salah intentionally until its time expires, he will never be able to make up for that salah. Such a person should turn to Allah and ask His forgivness and increase his good deeds and nawafil in order to increase his weight [of good] on the Day of Resurrection. Abu Hanifah, Malik, and ash-Shaf’i say that he can make up the prayer after its time has expired, and Malik and Abu Hanifa even say that if a person intentionally misses a prayer or a few prayers, then he is to make up those prayers before he prays the present salah, even if he has missed all five prayers and should, while making them up, miss the present salah. They say that if he missed more than five prayers, he is to begin by praying the salah whose time is present [and then he is to make up the prayers he missed]. The proof for our position [i.e, the position of Ibn Hazm] is found in the words of Allah, the Exalted: “Woe unto the worshippers who are heedless of their prayers,” and: “And then there succeeded them a later generation who wasted the prayers and followed their own lusts, but they will meet with destruction.” If one who intentionally misses a salah could make it up later, then why is it mentioned with affliction or transgression? Of course, there is no affliction or transgression on one who delays the salah. But the case of one who procrastinates until the last portion of its permissible time expires is quite different. Allah, the Exalted, has appointed certain times for the fard salah; both the beginning time and the ending time for the salah have been established, and there is no difference between praying a salah before its time and praying it after its proper time elapses because, in both cases the salah is not performed within its prescribed time. This is not to draw an analogy between one and the other but it is applying the same rule to them as they both must be performed within the limits set by Allah. Allah, the Exalted, says: “Whoever transgresses the limits set by Allah has verily wronged his own soul!” : The principle of making qada’ must be established by the proper sources of Islamic law. Legislating [in shari’ah] is not permissible, save by Allah’s authority as evidenced by His Prophet.
We ask those people who say that one may make qada’ for a salah which he misses intentionally: “Tell us about this salah that you want him to perfom, is it the same salah that Allah ordered him to perform or is it a different one?” If they say it is the same one, then we may say to them: “Then one who misses it intentionally is not guilty of being disobedient [to Allah, the Exalted], as he has done what Allah had ordered him to do, and there is no sin upon him according to your statement and likewise there should be no blame upon one who intentionally delays a salah until its time expires, but that is not an acceptable position for any Muslim.” If they say that it is not the salah which Allah ordered, we may say: “You have told the truth,” and this is a sufficient confession from them. Then, we may ask them: “Is one who intentionally leaves the salah until its time expires being obedient or disobedient to Allah?” If they say obedient, they will be differing from the consensus of the Muslims and the Qur’an and the confirmed sunnah. If they say he is being disobedient, they are speaking the truth and it is not valid that an act of disobedience should replace an act of obedience. Also, Allah, the Exalted, has set specific limits, through the tongue of His Messenger, for the times of the salah. Each salah has a specific beginning time, and no one may perform the salah before that time, and each prayer has a specific ending time, and no one may perform the salah after that time. No one of this ummah will dispute that point. However, if one is allowed to pray after the time set by the Messenger of Allah, then setting an ending time for the salah has no meaning to it. Such an opinion is nonsense and may Allah, the Exalted, keep us from it. Every action is connected with a certain time and it is not valid outside of that time; if it was valid outside of that time, what would be the purpose of that time being specifically singled out for that act? [The logic of this argument] is clear and Allah, the Almighty, is our Supporter.
Ibn Hazm discusses this point at great length, and adds: “If making up a salah is obligatory for one who has left a salah, even after its time has expired, why is it that Allah and His Messenger have chosen not to mention that fact as (surely) they did not forget it: “And your Lord is not forgetful!” Any law that is not based on the Qur’an or the sunnah is not valid. It has been authentically reported that the Prophet said: “Whoever misses the ‘asr salah, it is as if he has lost his family and his property.” It is correct to say that if one “misses” something, he cannot make it up, for if he makes it up or could make it up, the act would not be “missed.” The entire Muslim ummah is in agreement with the statement and ruling that if the time of the salah has elapsed, then the salah is “over” [i.e., “qada” in Arabic], but if one can make it up, the statement that the salah is “over” becomes false and untrue; therefore, there is no way that it could ever be made up. The people who agree with us on this include ‘Umar ibn al-Khattab, his son ‘Abdullah, Sa’d ibn Abi Waqas, Salman al-Farsi, ibn Mas’ud, al-Qasim ibn Muhammad ibn Abu Bakr, Budail al-‘Uqaili, Muhammad ibn Sireen, Mutraf ibn ‘Abdullah, ‘Umar ibn ‘Abdulaziz, and others. Allah has left no excuse, for anyone required to perform the salah, to delay the salah from its proper time for any reason whatsoever, not even during times of fighting, fear, extreme illness, or travelling. Allah says: “And when you are among them and arrange them for salah, let only one party be with you” And: “But if you are in danger, then walking or riding.” Allah, the Exalted, does not permit even the extremely sick person to delay the salah. In fact, such a person has been ordered to pray sitting, if he cannot pray standing, and if he cannot pray sitting, then he may pray on his side. Also, if one cannot make ablution with water, he may make tayammum; and if he cannot find soil to make tayammum, he may still pray. Whence has the permission been obtained that one may intentionally leave the salah until its time is finished and who has ordered that it be performed after its time and how is it that the belated salah would be sufficient? None of this is derived from the Qur’an, Sunnah, Qiyas (analogical reasoning), and so forth.
Ibn Hazm further says: “Concerning our statement that the one who intentionally leaves a salah until its time expires is to repent to Allah, the Exalted, ask for His forgiveness, pray an increased number of nawafil, and do good deeds. This statement is based on Allah’s words: “Then there succeeded them a generation who missed prayers and followed after lusts. But they will meet destruction save him who repents and believes and does right. Such will enter the garden and will not be wronged,” and: “…those who, when they do an evil thing or wrong themselves, remember Allah and implore forgiveness for their sins – who forgives sins, save Allah – and will not knowingly repeat the wrong they did,” and: “Whoever does an atom’s weight of good shall see it and whoever does an atom’s weight of evil shall see it,” and: “…We set a just balance for the day of resurrection so no soul shall be wronged.”
This [Muslim] ummah is in agreement, and there are texts that state that voluntary acts are a type of good deeds and Allah knows how much they are really worth. It necessarily follows that a number of voluntary works may be equivalent in merit to an obligatory deed and may even amount to a greater merit. Furthermore, Allah has informed us that He does not waste the action of any person and that the good deeds erase the evil ones.Imanway.com