Musnad Ahmad 5:100

Khalaf narrated to us, Qays narrated to us, from al-Ash'ath ibn Siwar, from 'Adi ibn Thabit, from Abu Zabyan, from 'Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "O 'Ali, if you are entrusted with the command after me, then expel the people of Najran from the Arabian Peninsula."