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