Musannaf Ibn Abi Shayba 33:593

Mu'adh narrated to us, from Ibn 'Awn, from Muhammad, who said: Abu Musa wrote to 'Amir: From 'Abdullah ibn Qays to 'Amir ibn 'Abdullah, who used to be called 'Amir ibn 'Abd Qays. To proceed: I knew you to be upon a certain matter, and it has reached me that you have changed. If you are upon what I knew you to be, then fear Allah and remain [steadfast]. And if you have changed, then fear Allah and return.