Shulchan Arukh, Orach Chayim 120/שולחן ערוך·אורח חיים ק״כ

1That It Is Proper To Say "R'tzei" in Every [Amidah] Prayer. Containing 1 S'if:We say "R'tzei" in all the [Amidah] prayers; and this is not like those who have a custom to not say it in the afternoon [Amidah] prayer.