To Gain and to Give: Hilchot Tzedakah, Day 25

It is a מצוה מן המובחר (punctilious performance of a mitzvah) to give to tzedakah a fifth of one’s earnings, in place of the standard ma’aser. Chazal prohibited giving more than a fifth out of concern that the giver could possibly become needy and dependent on others themselves (to be continued).

שו"ע ורמ"א ס' רמט ס"א, ש"ך סק"ב