Ba'al Shem Tov, Re'eh 11/בעל שם טוב על התורה·ראה י״א

1“One who wants to be generous, should not give more than a fifth.”That is, if he feels like he is stealing from himself. But if his heart trusts firmly that G-d will reimburse him, and he does not resent giving charity, he can give as much as he wants.Ma’aseh Yechiyel, VaYetze 14