Responsa of R. Moshe Feinstein, Yoreh Deah 1:143

And with what Usha enacted [Babylonian Talmud, Ketubot 50a] not to give more than one fifth [of one’s money] to charity, whether it is a prohibition or rather good advice, it seems from the text of the Talmud that it is a prohibition...Practically speaking, do not give more than one fifth if there is no life threatening need. [Translated by Josh Yuter]