As regard to the topic, following is the Fatwa issued by the eminent Muslim scholar, Sheikh Yusuf Al-Qaradawi, on combining prayers:
“Salah (prayer) is a religious duty that has specified times. Allah says: “Verily, the prayer is enjoined on the believers at fixed hours.” (An-Nisa’: 103)

these fixed hours have come to be recognized by way of following the actual practice of the Prophet (peace and blessings be upon him). So, each of the five prayers has its own time limit that should not be violated, whether by doing the salah ahead of it, which, is prohibited and categorically unacceptable, or by exceeding it and doing the salah after it unless there is an acceptable excuse. Otherwise, one commits a sin by so doing.

anyhow, it is out of this religion’s ease and practicality that it makes it permissible to combine two prayers, i.e., Zuhr and `Asr together and Maghrib and `Isha’ together, whether by offering the second at the time of the first or by delaying the first to the time of the second, on account of some reasons, including travel, as proven by the Sunnah of the Prophet (peace and blessings be upon him). Following are some other reasons accepted for such permissibility:

1. Rain, mud, snow, violent wind and storms, as well as any similar inclement weather conditions or natural disasters that prevent people from performing each salah on time except through formidable pain and trouble.

2. Need or fear. This is meant to remove pain and trouble from people, as indicated in the hadith, narrated by Ibn `Abbas. Imam Muslim narrated on the authority of Ibn `Abbas (may Allah be pleased with them both) that he said, “The Messenger of Allah (peace and blessings be upon him) has often combined Zuhr and `Asr, and Maghrib and `Isha’, for no fear or travel.”

another version of the hadith reads, “The Messenger of Allah (peace and blessings be upon him) combined Zuhr and `Asr, and Maghrib and `Isha’, in Madinah under no circumstances of fear or rain.” Ibn `Abbas was asked, “So, why did he do that?” He answered that he (the Prophet) wanted to remove hardship and troubles off his nation.

In the version of `Abdullah Ibn Shaqiq, it was stated that Ibn `Abbas one day delivered a sermon in the afternoon until the sun set and the stars began to appear in the sky, so people kept saying: “The prayer! The prayer!” And a man from the tribe of Tamim walked towards him and said vehemently: “The prayer! The prayer!” Ibn `Abbas replied: “Are you going to teach me the Sunnah? What a fool you are!” Then he said: “I’ve seen the Messenger of Allah (peace and blessings be upon him) combine Zuhr and `Asr, and Maghrib and `Isha’.” The narrator, `Abdullah Ibn Shaqiq, went on to say: “I felt uneasy, so I went to Abu Hurayrah and asked him about that and he confirmed the validity of what Ibn `Abbas said.” (Sahih Muslim)

this explanation from Ibn `Abbas means that the Prophet (peace and blessings be upon him) wanted to allow his nation more ease and facility rather hardship, since Allah the Almighty has not laid upon Muslims in religion any sort of hardship and He intends for them ease and does not want to make things difficult for them.

this hadith also stands as explicit evidence for the permissibility of combining two prayers for necessity.

Imam Ibn Sireen was reported as saying that he did not see any objection to doing so, as long as there is a need for it, unless people turn it into a habit. Likewise, Ibn Shibrima held the same opinion. In his Fath Al-Bari, Ibn Hajar said that many Imams — including Ibn Sireen, Rabi`a, Ash-hab, Ibn Al-Mundhir and Al-Qaffal Al-Qabir — held the opinion that the tradition in question may be applied literally; hence they saw it permissible for any Muslim to combine prayers while not traveling for whatever genuine need without restrictions, but on the condition of not taking it as a habit in itself.

the Hanbali jurists have permitted for the Muslim to combine prayers once in a while due to certain excuses. So, if there is a burden in praying the obligatory prayers at the appointed time, combining is allowed, on the condition that the person does not take it as habit or as something done every two or three days or whenever he wants to go out on different occasions, a few times a week or a number of times a month. But this is permitted only some of the time, not all of the time, so that it can remove the burdens and hardships which face the people.
For example, if a traffic policeman has his shift beginning before Maghrib and continues until after `Isha’, then he can combine Maghrib and Isha’. Similarly, if a doctor has to perform an urgent medical operation, he is allowed to make an early or delayed combination.”

An eminent scholar of Bahrain, Sheikh Nizam Ya`qubi, adds:
“Allah states clearly in the Qur’an that each prayer has its own prescribed time, “Verily, the prayer is enjoined on the believers at fixed hours.” (An-Nisa’: 103) So this means that each prayer must be performed at its own prescribed time.
however, this rule is relaxed in cases of necessity and ample need. For example, traveling, sickness, warfare, rain and bad weather, etc. I must emphasize here that this rule should be taken only in difficult situations and not to make it a regular habit. There are many hadiths which mention punishment for those who delay their prayers without acceptable reason.”