Musnad Ahmad 32:404

'Abd al-Rahman ibn Mahdi narrated to us, from 'Imran al-Qattan, from Qatadah, from Anas, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) appointed Ibn Umm Maktum as his successor over Medina twice. And I indeed saw him on the day of al-Qadisiyyah with a black banner in his possession."