Musnad Ahmad 30:1316

Umar ibn Sa’d, who is Abu Dawud al-Hafari, narrated to us, he said: Sufyan informed us, from al-Awza’i, from Yahya ibn Abi Kathir, from Abu Salamah, from Abu Hurayrah, who said: Food was brought to the Prophet, may Allah bless him and grant him peace, at Marr al-Zahran. He said to Abu Bakr and Umar: "Draw near, and eat." They said: "We are fasting." He said: "Prepare to depart for your two companions, work for your two companions."