סימן קכז סעיף טז

אֲבָל אִם אָכַל שֶׁלֹּא בְּהֶתֵּר בְּיוֹם הַתַּעֲנִית, בֵּין בְּשׁוֹגֵג בֵּין בְּמֵזִיד, חַיָּב לְהַשְׁלִים אֶת הַתַּעֲנִית גַּם לְאַחַר הָאֲכִילָה. וְיֵשׁ לוֹ לְהִתְעַנּוֹת אַחַר כָּךְ שֵׁנִי חֲמִישִּׁי וְשֵׁנִי לְכַפָּרָה עַל מַה שֶּׁאָכַל בְּיוֹם הַתַּעֲנִית. וּמִכָּל שֶׁכֵּן, אִם הָיָה יוֹם הַתַּעֲנִית מֵחֲמַת נֵדֶר, שֶׁצָּרִיךְ לְהַשְׁלִים נִדְרוֹ אַחַר כָּךְ (תקסט).

However, if you ate on a fast day in violation of the law, whether inadvertently or intentionally, you must complete the fast, even after having eaten. Afterwards, you must fast on Monday, Thursday and Monday, as an atonement for eating on the fast day.¹³This rule applies only if you ate a kazayis of food, but if you ate less than that, you need not fast another day as an atonement. Similarly, in regard to drinking, if you drank less than a cheekful, you may continue the fast, and it is not required that you fast on another day. (Ibid. 568:5) ¹⁴Mishnah Berurah makes no mention of the need to fast on Monday, Thursday and Monday as an atonement. It would seem that one day is sufficient as an atonement. It is certainly true that you must fulfil your vow at a later time if you had to fast because of a vow you had made.

תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)