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 ofRamadaan 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.

==========

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 Uthaymeen’s explanation of ‘Umdat-ul-Ahkaam
translated by Nasser ibn Najam)