Sahih Muslim | Book 6 | Hadith Number 2487

Narrated by 'Aisha (Allah be pleased with her)
'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.