Musnad Ahmad 333:6

'Abd al-Rahman ibn Mahdi narrated to us, he said: 'Ikrimah ibn 'Ammar narrated to us, from Iyas ibn Salamah, from his father, who said: "We spent the night [preparing to attack] Hawazin with Abu Bakr al-Siddiq, and the Prophet, may Allah bless him and grant him peace, had appointed him in command over us."