Sahih Muslim Book 6, Hadith Number 2488.

Chapter : Choice for observing fast and breaking it on a journey.

‘Aisha (Allah be pleased with her) reported that Hamza b. Amr al-Aslami asked the Messenger of Allah (may peace be upon him) 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.