As for the fast of a vow (nadhr), it is of three types. The first is that a person vows to fast for Allah سبحانه وتعالى, the Exalted, either a month or a specified number of days. In this case, the fast becomes obligatory upon him, but it is not permissible for him to fast it while travelling. The second is that he vows to fast on a specific day, and that day coincides with him being a traveller. Its ruling is the same as the first category: it is not permissible for him to fast it while travelling. The third is that he specifies the fast of a particular day, and stipulates upon himself that he will fast it both in travel and in residence. In that case, fasting that day becomes binding upon him while travelling, just as it becomes binding upon him while resident. What indicates the first category is what he narrated (in the following hadith):
Commentary #21