The Artscroll Machzor for Rosh Hashanah and Yom Kippur notes in the footnotes to U’nesaneh Tokef (p. 483 in the Rosh Hashanah one, p. 533 in the Yom Kippur one) that “the Chofetz Chaim used to say that if one is satisfied with the bare minimum of food, shelter, and clothing for his family, he may perhaps maintain a similar standard of charity. But if he desires more for himself, he must give more for the needy.”
Artscroll, however, leaves this statement unsourced. Does anyone know where the Chofetz Chaim actually says this?