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, inRamadaan.

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 Bassaam’s explanations of ‘Umdat-ul-Ahkaam
translated by Nasser ibn Najam)