Sahih Muslim Book 6, Hadith Number 2483.

Chapter : Permissibility of observing the fast or not observing it in the month of Ramadan for a traveller.

Abu Khalid al-Ahmar narrated from Humaid who said: I went out and was fasting; they said to me: Break (lit go back, repeat). He said that Anas reported that the Companions of the Messenger of Allah (may peace be upon him) used to set out on a journey and neither the observer of the fast found fault with the breaker of the fast, nor the breaker of the fast found fault with the observer of the fast. (One of the narrators Humaid said): I met Ibn Abi Mulaika who informed me the same thing on the authority of ‘Aisha.