Musnad Ahmad 788:201

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Sa'id ibn Abi Burdah, from his father, from Abu Musa, from the Prophet, peace and blessings of Allah be upon him, that he said: "Upon every Muslim is a charity." They said: "If he does not find [anything to give]?" He said: "He should work with his hands, so he benefits himself and gives charity." They said: "If he does not do that or is unable to?" He said: "He should help the needy, the distressed." They said: "If he is unable or does not do that?" He said: "He should enjoin good." They said: "If he is unable or does not do that?" He said: "He should withhold from evil, for it is a charity."