There are two kinds of Zuhd: Obligatory Zuhd and voluntary Zuhd. The former requires from you that you abandon pride, haughtiness, and arrogance; that you avoid doing deeds for show; that you stop trying to gain fame; and that you desist from adorning your deeds for other people. The latter involves forsaking the lawful things that Allah gave you. If you forsake some of the lawful things He gave you, it becomes obligatory upon you not to return to those lawful things unless you do so for the sake of Allah.

—Sufyān al-Thawrī [d. 161H/778CE]

(Read on pg 93, Salaahud-Deen ibn ‘Alee ibn ‘Abdul-Maujood, The Biography of Sufyaan Ath-Thauree. Darussalam Publishers. Riyadh:2005.)