Musannaf Ibn Abi Shayba 33:365

Abu Usama narrated to us, from Abd al-Hamid ibn Ja'far, who said: Salih ibn Abi Arib narrated to me, from Kathir ibn Murrah al-Hadrami, who said: I heard Abu al-Darda say: Shall I not inform you of the best of your deeds, and the most beloved of them to your King, and the one that raises your ranks the most? It is better for you than fighting your enemy, where they strike your necks and you strike their necks; it is better for you than giving away dinars and dirhams. They said: And what is it, O Abu al-Darda? He said: The remembrance of Allah is greater.