From Sahih Bukhari (Hadith 4279):
Narrated Tawus:Ibn `Abbas said, "Allah's Messenger (ﷺ) travelled in the month of Ramadan and he fasted till he reached (a place called) 'Usfan, then he asked for a tumbler of water and drank it by the daytime so that the people might see him. He broke his fast till he reached Mecca." Ibn `Abbas used to say, "Allah's Apostle fasted and sometimes did not fast while traveling, so one may fast or may not (on journeys)
Reference: Bukhari 4279