Sunan an-Nasa'i #2538 · The Book of Zakah

"Every Muslim must give charity." It was said: "What if he cannot find (anything to give)? "He said: "Let him work with his hands and benefit himself and give in charity." It was said: "What if he cannot do that?" He said: "Let him help someone who is in desperate need." It was said: "What if he cannot do that?" He said: "Let him enjoin good." It was said: "What if he cannot do that? He said: "Let him refrain from doing evil, for that is an act of charity