In the Qur'an, our Lord states the following about the issue:

The month of Ramadan, in which the Qur'an was sent down as guidance for people, and as clear signs of Guidance and the Criterion (between truth and falsehood). Therefore whoever of you is present this month must fast it, and whoever is so ill that he cannot fast or is on a journey (must fast the same) number of other days. God wills ease for you, and He does not will hardship for you...”The Qur'an, Al-Baqarah (The Cow) 2: 185

As is understood from the verse, if hardships will are to be experienced on a journey, then there is no drawback for a Muslim to eat his/her fast (not to fast) on condition that s/he fasts at another time as compensation. In a hadith (saying of the Prophet), we see that, at a time of hardships, the Prophet himself broke his fast (before due time) so that he could set an example for his community:

Jabir, may Allah be pleased with him, narrates:

In the year of conquest (of Mecca), Allah's Messenger, peace and blessings upon him, headed for Mecca and set out in the month of Ramadan.

He and those with him fasted until they came to the place known as Qura'u'l-Ghamim.

Then, he asked for a glass of water there and lifted the glass. Everybody looked at the glass. Then he drank of the water.

After that, some people informed him: “Some of the people fasted (continued to fast).”

Allah's Messenger (PBUH) said: “They are rebels! They are rebels!” (Muslim, Siyam: 90; Tirmidhi, Sawm: 18; Nasai, Sawm: 49)

Here saying that they are 'rebels' does not mean that they rebel against him. They are qualified so because they tormented their souls by persisting in a fast which would leave them weak at the time of an important campaign, under the scorching conditions of the desert.

In still another hadith, Prophet Muhammad (PBUH) said that those who don't fast at times when the circumstances are harsh, are advantageous:

Anas, may Allah be pleased with him, narrates:

We were together with Allah's Messenger (PBUH) in a campaign. Some among us were fasting and some not. On a hot day, we sojourned at some place. Most of those under the shade were the ones who had clothes on. Some of us were protecting ourselves from the sun with our hands.

At some time, those who were fasting collapsed on the ground, and those who weren't fasting got up, set the tents, and watered the animals.

Thereupon, Allah's Messenger (PBUH) said:

Today those who aren't fasting earned the thawab (reward for good deeds that is tallied on judgment day).” (Bukhari, Jihad: 71; Muslim, Siyam: 100; Nasai, Sawm: 52.)

In another similar hadith, Allah's Messenger (PBUH) said that it is not piety to stubbornly continue fasting under harsh circumstances:

Narrated by Jabir bin 'Abdullah:

Allah's Messenger (PBUH) was on a journey and saw a crowd of people, and a man was being shaded (by them). He asked, "What is the matter?" They said, "He (the man) is fasting." The Prophet said, "It is not berr (righteousness; piety) to fast on a journey."

(Bukhari, Sawm: 36, Muslim, Siyam: 92; Abu Dawud, Sawm: 43; Nasai, Sawm: 48.)

This is because in the verse above, our Lord says: “Allah wills ease for you, and He does not will hardship for you.” It is a sort of betrayal and torment to the blessing of body, which Allah granted to us, to drag ourselves into hardships. On every occasion, our Prophet set good examples of the conveniences that our religion grants to us:

Amr Ibn-i Umayya ad-Damri, may Allah be pleased with him, narrates:

Back from a journey, I visited Allah's Messenger (PBUH). He said to me:

“O Abu Umayya, wait for the morning meal (let us eat together).”

I replied: “I am fasting.”

“Then, come, listen to me, I will inform you about the traveler.” and he carried on:

Allah freed the traveler from fasting and from half the obligatory prayer. (Nasai, Sawm: 50)

Muhammad Ibn-i Ka'b narrates:

In Ramadan, I went near Anas Ibn-i Malik, may Allah be pleased with him. He was preparing for a journey. His camel was prepared and he wore his garments of journey. He sent for a meal and ate it. I asked him:

“Is it a sunnah (an act which the Prophet performed; not required but carries much reward) (to break the fast before setting out for a journey)?

He replied: “Yes!” and getting on his mount, he set off. (Tirmidhi, Sawm: 76)

In the following hadiths, we see that under the circumstances when hardships lessen, the Prophet permits fasting:

Aisha, may Allah be pleased with her, narrates:

Hamza Ibn-i Amr al-Aslami, may Allah be pleased with him, asked Allah's Messenger (PBUH) about fasting on a journey. He used to fast a lot.

Allah's Messenger (PBUH) answered:

If you want, fast; if not, then don't fast (It is up to you.) (Bukhari, Sawm: 33; Muslim, Siyam: 103; Muwatta, Siyam: 24; Tirmidhi, Sawm: 19; Abu Dawud, Sawm: 42)

Anas, may Allah be pleased with him, narrates:

We were together with Allah's Messenger (PBUH) (on a campaign). Some of us were fasting, and some not. Neither the fasting nor the non-fasting was reproaching one another.”

(Bukhari, Sawm: 37; Muslim, Siyam: 98; Muwatta: 23; Abu Dawud, Sawm: 42)

Abu Said al-Khudri, may Allah be pleased with him, narrates:

We would set off for journey in Ramadan with Allah's Messenger (PBUH). Neither the fasting nor the non-fasting was reproaching one another. (Muslim, Sıyam: 95; Nasâi, Sıyam: 59)

It brings more thawab for those who have the opportunity during a journey to choose to fast.  As is clearly understood from the examples above, the reason for breaking the fast before sunset during journey is the presence of hardship. Under the conditions when there is no such hardship, it would be appropriate for the believer to choose to fast.

An example about the subject from the Era of Bliss (The time of the Prophet):

Salama Ibn-i Muhabbaq, may Allah be pleased with him, narrates:

Allah's Messenger (PBUH) said:

Whoever reaches Ramadan on a journey and has a mount to bring him to the place where he will eat, let him fast wherever he is.(Abu Dawud, Sawm: 44)

In the Qur'an, in Al-Baqarah Surah, in the 184th verse, while it is stated that those who are ill and those who are on a journey may choose not to fast in Ramadan on condition that they compensate for it later, it is also stated: “...and that you should fast (when you are able to) is better for you, if you but knew (the worth of fasting)...”

To sum up, from those hadiths, the following rule is inferred:

As being on a journey is troublesome and hard, our religion granted us easiness in such worships as obligatory prayers and fasting.

On a journey, one performs the prayers that are four raka'ah (unit of Islamic prayer, or salat; each daily prayer is made up of a different number of raka'ah) by shortening; i.e. as two raka'ah.

Such a person (on a journey) is free to choose whether to fast or not. As is clearly stated in the verse, if s/he is to suffer a real trouble and hardship, s/he can choose not to fast on condition that s/he will compensate for it after Ramadan.

It is a religious permit and convenience. If there is the opportunity to fast with ease and if s/he does not suffer from any trouble, it is more virtuous and more rewarding for him/her to fast.

Today, the circumstances in journeys are much easier; journeys which would formerly take days, weeks or even months have been reduced to hours or even to minutes. Therefore, we should refrain from misusing that permit and should not break our fast before sunset if there is no hardship.