Musnad Ahmad 937:43

Sufyan narrated to us, Hisham ibn Urwah narrated to us, from his father, from Abu Murawah, from Abu Dharr, who said: I said: "O Messenger of Allah, which deed is best?" He said: "Faith in Allah and Jihad in His cause." I said: "O Messenger of Allah, which slaves are best [to free]?" He said: "Those most precious to their owners and highest in price." I said: "If I do not find [the means to do that]?" He said: "You assist a craftsman, or you work for one who is unskilled." I said: "And if I am not able?" He said: "Withhold your evil from the people, for it is a charity that you bestow upon your own self."