Fasting: Travelling Before or After Zuhr While Fasting
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1690
If a fasting person travels after ẓuhr, he must, based on obligatory precaution, complete his fast; and in such a case, it is not necessary for him to make up that fast. If he travels before ẓuhr, then based on obligatory precaution, he cannot fast on that day, particularly if he had made the intention to travel the night before. In any case, he must not do anything that invalidates a fast before reaching the permitted limit (ḥadd al‑tarakhkhuṣ);[1] otherwise, kaffārah becomes obligatory for him.
-Ayatullah Sistani, Practical Laws of Islam, Fasting (Sawm)
1
The permitted limit is explained in Ruling 1304.