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