‘A’isha (Allah be pleased with her) reported that Hamza b. Amr al-Aslami asked the Messenger of Allah (ﷺ) thus: Messenger of Allah, I am a person devoted much to fasting. Should I fast during the journey? He (the Holy Prophet) said: Fast if you like and break it if you like.
وَحَدَّثَنَا أَبُو الرَّبِيعِ الزَّهْرَانِيُّ، حَدَّثَنَا حَمَّادٌ، – وَهُوَ ابْنُ زَيْدٍ – حَدَّثَنَا هِشَامٌ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، – رضى الله عنها – أَنَّ حَمْزَةَ بْنَ عَمْرٍو الأَسْلَمِيَّ، سَأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم فَقَالَ يَا رَسُولَ اللَّهِ إِنِّي رَجُلٌ أَسْرُدُ الصَّوْمَ . أَفَأَصُومُ فِي السَّفَرِ قَالَ “ صُمْ إِنْ شِئْتَ وَأَفْطِرْ إِنْ شِئْتَ ” .
Reference : Sahih Muslim 1121b
In-book reference : Book 13, Hadith 132
USC-MSA web (English) reference : Book 6, Hadith 2488