Musnad Ahmad 1302:13

Hajjaj ibn Muhammad narrated to us, Abu Ma'shar narrated to us, from Musa ibn 'Uqbah, from Ziyad ibn Abi Ziyad, the freed slave of Ibn 'Ayyash, from Abu al-Darda', who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Shall I not inform you of the best of your deeds, the purest of them with your Sovereign, the highest of them in your ranks, and what is better for you than giving gold and silver, and better for you than that you encounter your enemy and strike their necks and they strike your necks?" They said: "Yes." He said: "The remembrance of Allah, the Almighty and Majestic."