Musnad Ahmad 788:267

Yazid ibn Harun narrated to us, he said: Al-Awwam informed us, and Muhammad ibn Yazid—the meaning is the same—said: Al-Awwam narrated to us, he said: Ibrahim Abu Isma'il al-Saksaki narrated to me, he said: I heard Abu Burdah ibn Abi Musa while he was saying to Yazid ibn Abi Kabshah—and they were companions on a journey, and Yazid used to fast during the journey—so Abu Burdah said to him: I heard Abu Musa say repeatedly: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Indeed, when the Muslim 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." Muhammad—meaning Ibn Yazid—said: Allah writes for him the like of what he used to do while resident and healthy.