It is obligatory to offer the prayers on time as enjoined by Allah, may He be exalted, and it is not permissible to delay them except in the case of an excuse that make it permissible to put prayers together. Allah, may He be exalted, says (interpretation of the meaning):
Verily, the prayer is enjoined on the believers at fixed hours
Being careless about the prayer and delaying it until the time for it is over without any excuse is a major sin, because Allah, may He be exalted, says (interpretation of the meaning):
Then, there has succeeded them a posterity who have given up As-Salat (the prayers) (i.e. made their Salat (prayers) to be lost, either by not offering them or by not offering them perfectly or by not offering them in their proper fixed times, etc.) and have followed desires. So they will be thrown in Hell
Ibn Masood (may Allah be pleased with him) said concerning the word ghayy (translated here as Hell): This is a valley in Hell which is very deep and foul.
The Prophet (blessings and peace of Allah be upon him) said: Whoever fails to pray Asr, his good deeds will be rendered invalid. Narrated by al-Bukhaari, 553.
With regard to putting together Zuhr and Asr, and Maghrib and Isha', at the time of the earlier or later prayer, that is permissible in the case of excuses that have been defined by the scholars, one of which is so as to alleviate hardship and difficulty. This is appropriate in the case mentioned here. If the doctor is afraid that if the surgery takes a long time and when he has finished, the time for Isha' prayer, for example, will have ended, then he may put Maghrib and Isha' together at the time of the earlier prayer, and offer them at the time of Maghrib. If he is going to start surgery before the time for Maghrib begins, and he will finish before the end of the time for Isha', then he may delay Maghrib and put it together with Isha' at the time of the later prayer, after he has finished doing surgery. The same applies with regard to Zuhr and Asr.
The evidence for that is the report narrated by Muslim (705) from Ibn Abbaas (may Allah be pleased with him) who said: The Messenger of Allah (blessings and peace of Allah be upon him) put together Zuhr and Asr, and Maghrib and Isha', in Madeenah when there was no fear and no rain. According to the hadeeth of Wakee he said: I said to Ibn Abbaas: Why did he do that? He said: So that his ummah would not be subjected to hardship.
Whenever there is difficulty and hardship, it is permissible to put together Zuhr and Asr, and Maghrib and Isha'. With regard to Fajr, it cannot be put together with anything else before or after.
Prayer is extremely important, and it is a serious matter. It is the first thing for which the individual will be brought to account, so it is essential to pay attention to it and offer prayer regularly and on time.
We ask Allah to help and guide you.
And Allah knows best.
Reproduced from Islam QA