Sahih Muslim Book 6, Hadith Number 2476.
Chapter : Permissibility of observing the fast or not observing it in the month of Ramadan for a traveller.
This Hadith has been narrated on the authority of Shu’ba with the same chain of transmitters but with this addition that he (the Holy Prophet) said: “Take advantage of the concession of Allah Who Wanted it to you.” When he (one of the narrators) asked him (the other one, Yahya b. Abi Kathar) he did not retain it in his mind.