Rules of Travelling in the Day when he has Vowed to Observe fast
Issue No. 2285- If one vows that he will observe fast on a particular day, the obligatory precaution is that he should not go on a journey on that day so that he can observe fast. However, if he travels on that day, it is obligatory to perform that fast as qaḍā later, and he should also offer kaffāra for it as an obligatory precaution.