Hadith: We set out during the month of Ramadan with the Messenger of Allah (may Allah's peace and blessings be upon him) in such intense heat that one of us would place his hand over his head (to protect himself) against the excessive heat, and none of us was fasting except the Messenger of Allah (may Allah's peace and blessings be upon him) and ‘Abdullāh ibn Rawahah

عن أَبِي الدَّرْدَاءِ رضي الله عنه قال: «خَرَجْنَا مع رسول الله صلى الله عليه وسلم في شهر رمضان، في حَرٍّ شَدِيدٍ ، حتى إن كان أَحَدُنَا لَيَضَعُ يَدَهُ على رأسهِ من شِدَّةِ الْحَرِّ. وما فِينَا صائمٌ إلا رسولُ الله صلى الله عليه وسلم وعبد ُالله بن رَوَاحَةَ».
[صحيح] - [متفق عليه]

Abu Ad-Dardā' (may Allah be pleased with him) reported: We set out during the month of Ramadan with the Messenger of Allah (may Allah's peace and blessings be upon him) in such intense heat that one of us would place his hand over his head (to protect himself) against the excessive heat, and none of us was fasting except the Messenger of Allah and ‘Abdullāh ibn Rawahah."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

Abu Ad-Dardā' (may Allah be pleased with him) reported that they set out on a journey during the month of Ramadan, and the weather was extremely hot that they would put their hands on their heads to protect them from the heat. None of them was fasting except the Prophet (may Allah's peace and blessings be upon him) and ‘Abdullāh ibn Rawahah al-Ansāri (may Allah be pleased with him). They both were able to fast and endure severity of the weather, and this is a proof that it is permissible for the Muslim to fast when traveling even with hardship or difficulty that does not lead to one's death.