Because Allah, the Exalted, has said (in Surah Al-Baqarah) «يا أيها الذين آمنوا أنفقوا من طيبات ما كسبتم‏» "O you who believe! Spend of the good things which you have (legally) earned, and of that which We have produced for you from the earth." Until the end of the verse «غني حميد‏» .

Narrated Abu Burda: from his father from his grandfather that the Prophet said, "Every Muslim has to give in charity." The people asked, "O Allah's Prophet! If someone has nothing to give, what will he do?" He said, "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 that?" He replied, "Then he should perform good deeds and keep away from evil deeds and this will be regarded as charitable deeds."