The Book of Fasting (Kitab Al-Sawm)
Muslim :: Book 6 : Hadith 2488
'A'isha (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.