Sahih Muslim Book 6, Hadith Number 2487.
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) about fasting on a journey, and he (the Holy Prophet) said: Fast if you like and break it if you like.