Shulchan Arukh, Orach Chayim 598/שולחן ערוך·אורח חיים תקצ״ח

1When Rosh HaShanah falls on Shabbat we say Tzidkatkhah at mincha. Rema: And there are those who say we do not say it [Ran end of Tractate Rosh HaShana and Minhagim], and this is our custom.