From Jaabir ibn ‘Abdillaah  radi Allaahu ‘anhu who said: 
 
The Messenger of Allaah sall Allaahu ‘alaihi wa sallam was upon a journey. Then he saw a crowd  and (he saw) a man who was being shaded. 
So he said, “What is this?”
They said, “A fasting person.”
He said, “Fasting upon a journey is not from righteousness.”
 
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The Shaykh Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah commented:
 
Jaabir radi Allaahu ‘anhu informed that the Prophet sall Allaahu ‘alaihi wa sallam was upon his journey in the year of the Conquest, in Ramadaan. 
 
So he saw a group crowding around, in order to take a look at a person  - as though he were affected by heat and thirst and he was being shaded.
 
And he was lying down, as occurs in the narration of ibn Jareer.
 
So the Prophet sall Allaahu ‘alaihi wa sallam asked (about him), so they said, “He is a man who is fasting.” 
 
And because the fasting had caused him to reach this degree (of discomfort), the Prophet sall Allaahu ‘alaihi wa sallam said that fasting upon a journey was not from righteousness when it caused the fasting person to reach that level (of discomfort).
 
And the Shaykh ‘Abdullaah ibn ‘Abdir Rahmaan aale Bassaam rahimahullaah commented:
 
And He (Allaah) did not wish from you that worship of Him would be torturing yourselves.





(taken from Sh Uthaymeen's and Sh Bassam's explanation of 'Umdat-ul-Ahkaam
translated by Nasser ibn Najam)

From Aboo Dardaa radi Allaahu ‘anhu who said: 
 
We set out (on a journey) with the Messenger of Allaah  sall Allaahu ‘alahi wa sallam in the month of Ramadaan in the severe blazing heat of the Sun to the extent that one of us would place (the palm) of his hand on his head (to protect against the Sun) due to the severity of the heat.
 
And there was no one fasting from amongst us except the Messenger of Allaah sall Allaahu ‘alaihi wa sallam and ‘Abdullaah ibn Rawaahah.
 
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The Shaykh ‘Abdullaah ibn ‘Abdir Rahmaan aale Bassaam rahimahullaah commented:
 
The Prophet sall Allaahu ‘alaihi wa sallam set out with his Companions in Ramadaan in days of extreme heat.
 
So from the severity of the heat, there was no one fasting from amongst them except the Prophet sall Allaahu ‘alaihi wa sallam and ‘Abdullaah ibn Rawaahah, the Ansaari radi Allaahu ‘anhu.
 
So these two people were able to endure the severe (conditions) and they both fasted. 
 
This is from (those matters) that prove the permissibility of fasting upon a journey, even if that is accompanied by hardship which does reach the level of (causing) destruction.







(taken from Sh Bassam's explanation of 'Umdat-ul-Ahkaam
translated by Nasser ibn Najam)

From Anas ibn Maalik radi Allaahu ‘anhu who said:
 
We would go on a journey with the Prophet  sall Allaahu ‘alahi wa sallam and the fasting person would not criticise the non-fasting person nor would the non-fasting person criticise the fasting one.
 
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The Shaykh ‘Abdullaah ibn ‘Abdir Rahmaan aale Bassaam rahimahullaah commented:
 
The Companions would go on a journey with the Prophet sall Allaahu ‘alaihi wa sallam so some of them would refrain from fasting and some of them would fast. 
 
And the Prophet sall Allaahu ‘alaihi wa sallam would consent to that because fasting is the asl (enduring normal rule), and refraining from fasting is the concession (to that rule).
 
And there is to be no criticism for leaving off the concession.
 
Therefore some of them did not criticise the others for fasting, or refraining from fasting.







(taken from Sh Bassam's explanation of 'Umdat-ul-Ahkaam
translated by Nasser ibn Najam)

From ‘Aaishah radi Allaahu ‘anhaa that Hamzah ibn ‘Amr al Aslamee radi Allaahu ‘anhu said to the Prophet sall Allaahu ‘alahi wa sallam:


Should I fast upon the journey?


And he was a person who was plentiful in fasting.


So he said, “If you wish, then fast. And if you wish, then abstain from fasting.”


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The Shaykh ‘Abdullaah ibn ‘Abdir Rahmaan aale Bassaam rahimahullaah (who died in 2003 CE) commented:

The Companions radi Allaahu 'anhum knew that the Merciful Legislator did not give the concession to refrain from fasting upon a journey except as a mercy to them and out of compassion for them. 

So Hamzah al Aslamee had steadfastness and strength upon fasting and he was one who loved al khayr (goodness), and was plentiful in fasting radi Allaahu ‘anhu . So he asked the Messenger of Allaah sall Allaahu ‘alaihi wa sallam, “Should I fast upon the journey?” 

So the Prophet sall Allaahu ‘alaihi wa sallam gave him the choice between fasting and refraining from fasting, saying, “If you wish to, then fast. And if you wish, then abstain from fasting.”




(taken from Sh Bassam's explanation of 'Umdat-ul-Ahkaam
translated by Nasser ibn Najam)