‘Abdullah b. Aun set two slaves free because once, when his mother called him, he raised his voice above hers.

Adh-Dhahabi, Siyar a’laam an-Nubalaa: 6/366

(Read on pg 63, Abdul Aziz ibn Nasir al-Jalil and Baha Addin ibn Fatah Aqeel, Wisdom of the Wise. Al-Firdous Ltd. London:2002 .)