Musnad Ahmad 788:195

Yazid narrated to us, he said: Al-Awam ibn Hawshab informed us, Ibrahim ibn Ismail al-Saksaki narrated to us, that he heard Abu Burdah ibn Abi Musa, and he and Yazid ibn Abi Kabshah accompanied each other on a journey, and Yazid used to fast. Abu Burdah said to him: I heard Abu Musa say repeatedly: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the servant becomes ill or travels, there is written for him of the reward the like of what he used to do while resident and healthy."