The Prophet Muhammad (peace be upon him) said: "Every Muslim has to give in charity." The people then asked: "(But what) if someone has nothing to give, what should he do?" The Prophet replied: "He should work with his hands and benefit himself and also give in charity (from what he earns)." The people further asked: "If he cannot find even that?" He replied: "He should help the needy who appeal for help." Then the people asked: "If he cannot do (even) that?" The Prophet said finally: "Then he should perform good deeds and keep away from evil deeds, and that will be regarded as charitable deeds." - Sahih Al-Bukhari, Volume 2, Hadith 524
The Prophet Muhammad (peace be upon him) said: "Charity is prescribed for each descendant of Adam every day the sun rises." He was then asked: "From what do we give charity every day?" The Prophet answered: "The doors of goodness are many...enjoining good, forbidding evil, removing harm from the road, listening to the deaf, leading the blind, guiding one to the object of his need, hurrying with the strength of one's legs to one in sorrow who is asking for help, and supporting the feeble with the strength of one's arms--all of these are charity prescribed for you." He also said: "Your smile for your brother is charity." - Fiqh-us-Sunnah, Volume 3, Number 98