Sahih Muslim , Book 6, Hadith 2488 (USC-MSA) ⓘ
'A'isha (Allah be pleased with her) reported that Hamza b. Amr al-Aslami asked the Messenger of Allah 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 " .
also referenced under: Book 13, Hadith 132 (In-Book) & Hadith 1121 (Reference)