Siman 563
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א מִי שֶׁהִרְהֵר בְּלִבּוֹ, שֶׁלֹּא בִּשְׁעַת תְּפִלַּת הַמִּנְחָה, לְהִתְעַנּוֹת לְמָחָר, לֹא הָוֵי קַבָּלָה דְּלֹא עָדִיף מֵהוֹצִיא בְּפִיו לְהִתְעַנּוֹת לְמָחָר דְּלֹא הָוֵי קַבָּלַת תַּעֲנִית, כֵּיוָן שֶׁלֹּא קִבְּלוֹ בִּתְפִלַּת הַמִּנְחָה.
1 If 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.