Musnad Ahmad 2:61

Hassan ibn Musa narrated to us, Ibn Lahi'ah narrated to us, he said: Yazid ibn Abi Habib narrated to us, from Ma'mar: That he asked Sa'id ibn al-Musayyib about fasting while traveling. So he narrated to him from Umar ibn al-Khattab that he said: We went on two military expeditions with the Messenger of Allah, may Allah bless him and grant him peace, during the month of Ramadan: the day of Badr and the day of the Conquest (of Makkah), and we broke our fast during both of them.