Maliks Muwatta Book 15, Hadith Number 24.
Section : Dhikr (Remembrance) of Allah, the Blessed and Exalted.
Yahya related to me from Malik that Ziyad ibn Abi Ziyad said that Abu’d-Darda had said, “Shall I not tell you the best of your deeds, and those that give you the highest rank, and those that are the purest with your King, and are better for you than giving gold and silver, and better for you than meeting your enemy and striking their necks?” They said, “Of course.” He said, “Remembrance (dhikr) of Allah ta ala.”