The traveller is allowed to combine between two prayers only when there is a valid excuse to do so, which would occlude his ability to perform each ṣalāh at its appropriate time. For example, because he is continuously travelling and is not able to stop or if there is difficulty in pausing. If, however, he stays in one place during his travel for two or three days, or even a day, then there is no valid reason to combine the performance of two prayers together. As, in this circumstance, he is fully able to perform each individual ṣalāh at its appropriate time without difficulty. Combining prayers when travelling is not a prespecified sunnah. Most travellers surmise that combining is a sunnah of travelling regardless of whether there is a valid excuse or not. Rather, combining prayers together has been permitted temporarily when travelling [in connection to a valid excuse]. Shortening the prayer is a prespecified sunnah— it is sunnah for the traveller to shorten any ṣalāh that has four rakʿahs, regardless of whether he has an excuse or not. As for him combining the performance of two prayers, it is done only when there is a clear need that would permit such an act. So this [combining two prayers] is one issue, and shortening is another.

Source: Al-Wābil al-Ṣayyib: 27-28
Translated by: Riyāḍ al-Kanadī