The Official Website of Grand Ayatollah Makarem Shirazi

The fast of the travelers who are obligated to perform prayers in Qasr form becomes invalidated, whereas those obligated to perform prayer in its complete form, even while on a journey such as Kathīr as-Safar, frequent traveler, must fast on a journey. Traveling during the month of Ramadan is not unlawful. However, traveling in order to escape fasting is disliked. Furthermore, if one makes an intention to pass the traveling limit on every day of Ramadan in order to break their fast, they are considered to be a frequent traveler and must observe all of their fasts. If one does not know that a traveler's fast is invalid, and finds out after Maghrib that they should not have fasted on a journey, their fast is valid. But if they find this out during the day, their fast is void. Also, if one forgets that they are a traveler, or that a traveler's fast is invalid and fast on a journey, then in both cases, based on obligatory precaution, their fast is invalid and they must perform the qada of the fast. However, not every journey invalidates fasting. If one makes the intention to stay somewhere for 10 days, they must fast, and even if one decides one day in the afternoon to come back from the journey before 10 days are completed, the fasts observed during the journey until that day are valid, but they must not fast in the rest of the journey. Likewise, when one observing fast sets out on a journey after Ẓuhr Adhān, their fast is valid, and they must not break it, but if they set out on a journey before midday and they are still on a journey at the time of Ẓuhr Adhān, their fast is invalid. However, they must be careful not to break the fast before reaching the permitted limit. Had tarakhkhuṣ If one breaks their fast before reaching the permitted limit, not only should they make up for the fast of that day, but giving kaffārah also becomes obligatory upon them, unless they had forgotten at that moment that they were fasting, or that they were not allowed to break their fast before reaching the permitted limit. Or they do not know at all that fast must not be broken before reaching the permitted limit, in which case, kaffārah does not become obligatory. Moreover, if one arrives in their hometown after Ẓuhr Adhān, they cannot fast.