Musnad Ahmad 31:252

Ibrahim ibn Ishaq narrated to us, Ibn al-Mubarak narrated to us, from Sa'id ibn 'Abd al-'Aziz, from 'Atiyyah ibn Qays, from Qaza'ah, from Abu Sa'id al-Khudri, who said: "When the Messenger of Allah, may Allah bless him and grant him peace, reached Marr al-Zahran in the year of the Conquest, he informed us of the meeting with the enemy, and he commanded us to break the fast, so we all broke our fast."