Fast if you like and break it if you like.
Sahih Muslim Book 13, Hadith 131
← Previous Hadith I came to Abu Sa'id al-Khudri (Allah be pleased with him) and he was surrounded (by people), and when they dispersed I said to him: I am not going to ask you about what these people were asking. I ask you about fasting on a journey. Upon this he said: We travelled with the Messenger of Allah (ﷺ) towards Mecca and we had been observing fast. We halted at a place. There the Messenger of Allah (ﷺ) said: You are nearing your enemy and breaking of fast would give you greater strength, and that was a concession (given to us). But some of us continued to observe the fast and some of us broke it. We then got down at another place and he (the Holy Prophet) said: You are going to encounter the enemy in the morning and breaking of the fast would give you strength, so break the fast. As it was a point of stress, so we broke the fast. But subsequently we saw ourselves observing the fast with the Messenger of Allah (ﷺ) on a journey.

Sahih Muslim Book 13, Hadith 130 Next Hadith → 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.

Sahih Muslim Book 13, Hadith 132