Musnad Ahmad 937:211

Yahya ibn Sa'id narrated to us, Hisham narrated to us, my father narrated to me, that Abu Marwah al-Ghifari informed him that Abu Dharr informed him that he said: "O Messenger of Allah, which deed is best?" He said: "Faith in Allah and Jihad in His path." He said: "Then which of the necks (slaves) is best?" He said: "The most expensive of them in price, and the most precious of them to their owners." He said: "What do you see if I do not do [that]?" He said: "You assist a craftsman, or you work for one who is unskilled." He said: "What do you see if I am weak?" He said: "You restrain yourself from evil, for it is a charity that you bestow upon yourself."