The World Federation One Stop Fiqh

Ruling 1686

If a person makes a vow to keep a recommended fast but does not assign a day for it, he cannot keep that fast on a journey. However, in the event that one makes a vow that he will fast on a particular day on a journey, he must keep that fast on a journey. Furthermore, if one makes a vow to fast on a particular day, whether he is travelling or not, he must fast on that day even if he is travelling.