Musannaf Ibn Abi Shayba 28:398

Zayd ibn al-Hubab narrated to us, he said: Mu'awiyah ibn Salih narrated to us, he said: Rabi'ah ibn Yazid al-Dimashqi narrated to me, from Qaza'ah, who said: I asked Abu Sa'id about fasting while traveling. He said: We traveled with the Messenger of Allah (may Allah bless him and grant him peace), and he would fast and we would fast, until we descended at a stopping place. He then said: "You have drawn near to your enemy, and breaking the fast is more strengthening for you."