Rambam, Mishneh Torah, Laws of Gifts to the Poor 7:5

If a poor person comes and asks according to his or her need and the giver cannot afford to give, the giver should give according to his ability. And how much is that? The greatest way to fulfill this commandment is to give up to one fifth of your wealth. The average way to fulfill this commandment is to give 10% of your wealth. Less than this is considered an evil eye. One should never give less than 1/3 of a shekel in a year, and anyone who gives less than this has not fulfilled the commandment. Even a poor person who is supported by tzedakah is obligated to give tzedakah to another. [AJWS translation]