Sahih Bukhari Volume 4, Book 52, Hadith Number 239.

Narated By Ibrahim Abu Isma’il As-Saksaki : I heard Abu Burda who accompanied Yazid bin Abi Kabsha on a journey. Yazid used to observe fasting on journeys. Abu Burda said to him, “I heard Abu Musa several times saying that Allah’s Apostle said, ‘When a slave falls ill or travels, then he will get reward similar to that he gets for good deeds practiced at home when in good health.”