If someone [having no legitimate excuse (ʿudhr)] is indecisive as to whether or not to invalidate an assigned [i.e. time-specific] obligatory fast – such as the fast of the month of Ramadan – or he decides to invalidate his fast [but does not do anything to break his fast] and does not make the intention to fast again, his fast becomes invalid.
If he does make the intention to fast again, the obligatory precaution is that he must complete the fast of that day and afterwards keep a qaḍāʾ fast for it.