Al-Imaam Ahmad Ibn Hanbal said: Az-Zuhd is of three types:
The first: abandoning that which is haraam (forbidden) – and it is the zuhd of the general folk.
The second: abandoning the excesses of that which is halaal (lawful) – and it is the zuhd of the people of distinction.
The third: abandoning that which busies one from Allaah – and it is the zuhd of the acquainted ones.
Imaam Ahmad Ibn Hanbal
Jaami' al-aadaab vol 1 page 372-373
Translated by Aboo Haatim Muhammad Farooq