Shulchan Arukh, Orach Chayim 563/שולחן ערוך·אורח חיים תקס״ג

1If one mentally expresses a plan to fast the following day not during the afternoon prayer, this does not constitute acceptance [of a fast], is it is not better than the verbal expression of a plan to fast the following day, which is not an acceptance of a fast because it is not during the afternoon prayer.