 Complete the following phrase:  As the nisba in his name suggests, he was a Khazar by origin, and is said to have been a slave working in the kitchen of Sallam al-Abrash al-Khadimwhence his nickname al-Tabbakh, the cookbefore he was purchased as a ghulām by al-Mutasim in 815. He rose to become one of the senior commanders in al-Mutasims Turkic guard, and participated in several expeditions such as the 