Sahih Muslim Book 5, Hadith Number 2202.

Chapter : Concerning one who spends and who withholds.

Sa’id b. Abu Burda reported on the authority of his grandfather that the Apostle of Allah (may peace be upon him) said: Giving of Sadaqa is essential for every Muslim. It was said (to him): What do you say of him who does not find (the means) to do so? He said: Let him work with both his hands, thus doing benefit to himself and give Sadaqa. It was said to him: What about him who does not have (the means) to do so? He said: Then let him assist the needy, the aggrieved. It was said: What do you say of one who cannot even do this? He said: Then he should enjoin what is reputable or what is good. He said: What about him if he cannot do that? He (the Holy Prophet) said: He should then abstain from evil, for verify that is Sadaqa on his behalf.