Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2487
'A'isha (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.