Fast if you like and break it if you like.  

أَنَّهَا قَالَتْ سَأَلَ حَمْزَةُ بْنُ عَمْرٍو الأَسْلَمِيُّ رَسُولَ اللَّهِ ﷺ عَنِ الصِّيَامِ فِي السَّفَرِ فَقَالَ إِنْ شِئْتَ فَصُمْ وَإِنْ شِئْتَ فَأَفْطِرْ  

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.  

أَنَّ حَمْزَةَ بْنَ عَمْرٍو الأَسْلَمِيَّ سَأَلَ رَسُولَ اللَّهِ ﷺ فَقَالَ يَا رَسُولَ اللَّهِ إِنِّي رَجُلٌ أَسْرُدُ الصَّوْمَ أَفَأَصُومُ فِي السَّفَرِ قَالَ صُمْ إِنْ شِئْتَ وَأَفْطِرْ إِنْ شِئْتَ  

This hadith has been narrated on the authority of Hisham with the same chain of transmitters.  

عَنْ هِشَامٍ بِهَذَا الإِسْنَادِ مِثْلَ حَدِيثِ حَمَّادِ بْنِ زَيْدٍ إِنِّي رَجُلٌ أَسْرُدُ الصَّوْمَ  

I am a person much used to fasting. Should I fast during the journey? (The rest of the hadith is the same.)  

إِنِّي رَجُلٌ أَصُومُ أَفَأَصُومُ فِي السَّفَرِ  

Messenger of Allah, I find strength in me for fasting on a journey; is there any sin upon me (in doing it)? Thereupon the Messenger of Allah ﷺ said: It is a concession from Allah. He who took advantage of it, it is good for him, and he who preferred to observe fast, there is no sin upon him. Harun (one of the narrators) in his narration said: 'lt is a concession, and he made no mention of" from Allah".  

أَنَّهُ قَالَ يَا رَسُولَ اللَّهِ أَجِدُ بِي قُوَّةً عَلَى الصِّيَامِ فِي السَّفَرِ فَهَلْ عَلَىَّ جُنَاحٌ فَقَالَ رَسُولُ اللَّهِ ﷺ هِيَ رُخْصَةٌ مِنَ اللَّهِ فَمَنْ أَخَذَ بِهَا فَحَسَنٌ وَمَنْ أَحَبَّ أَنْ يَصُومَ فَلاَ جُنَاحَ عَلَيْهِ قَالَ هَارُونُ فِي حَدِيثِهِ هِيَ رُخْصَةٌ وَلَمْ يَذْكُرْ مِنَ اللَّهِ  

We set out during the month of Ramadan with the Messenger of Allah ﷺ in such an intense heat that one of us would place his hand over his head (in order to protect himself) against the excessive heat, and none among us was observing the fast, except the Messenger of Allah ﷺ and 'Abdullah b. Rawaha.  

خَرَجْنَا مَعَ رَسُولِ اللَّهِ ﷺ فِي شَهْرِ رَمَضَانَ فِي حَرٍّ شَدِيدٍ حَتَّى إِنْ كَانَ أَحَدُنَا لَيَضَعُ يَدَهُ عَلَى رَأْسِهِ مِنْ شِدَّةِ الْحَرِّ وَمَا فِينَا صَائِمٌ إِلاَّ رَسُولُ اللَّهِ ﷺ وَعَبْدُ اللَّهِ بْنُ رَوَاحَةَ  

We were with the Messenger of Allah ﷺ on some of his journeys on an intensely hot day so much so that a person would place his hand on his head (in order to protect himself) against excessive heat, and none amongst us was fasting but the Messenger of Allah ﷺ and Abdullah b. Rawaha  

أَبُو الدَّرْدَاءِ لَقَدْ رَأَيْتُنَا مَعَ رَسُولِ اللَّهِ ﷺ فِي بَعْضِ أَسْفَارِهِ فِي يَوْمٍ شَدِيدِ الْحَرِّ حَتَّى إِنَّ الرَّجُلَ لَيَضَعُ يَدَهُ عَلَى رَأْسِهِ مِنْ شِدَّةِ الْحَرِّ وَمَا مِنَّا أَحَدٌ صَائِمٌ إِلاَّ رَسُولُ اللَّهِ ﷺ وَعَبْدُ اللَّهِ بْنُ رَوَاحَةَ