Musannaf Ibn Abi Shayba 28:269

Hushaym ibn Bashir narrated to us, from Hajjaj, from ‘Ata’ and al-Qasim ibn Abi Bazzah, from Mujahid; And from Layth, from ‘Ata’, from Mujahid; and from Abu Murrah, from al-Hasan; and Juwaybir, from al-Dahhak; they said: The Imam has the choice regarding the belligerent (muharib).