Imam Ibn Baaz said:
“It’s legislated for the traveller to shorten the prayer, he prays two Rakaats [in] Dhuhr, Asr and Isha. Two Rakats, this is what is better.
This is the Sunnah for the one who has travelled. It’s also permissible for him to join between Dhuhr and Asr [as one affair], and Maghrib and Ishaa [as another affair] but leaving it off [i.e. joining] is better if he has resided/descended and there is no hardship upon him. Leaving it off is better. As for if there is hardship and he has descended, then he combines.
The Prophet صلى الله عليه و سلم combined in his travels and [other times] left it off. At times, he would combine and at times he would leave it off صلى الله عليه و سلم. In the farewell Hajj, he didn’t combine while he descended in Mina.
He prayed every prayer in its proper time while he was in Mina. Thus if a person has descended for two or three days while in his travel, then that which is better is not to combine except if there is hardship in it for him and his companions. Then [in this case] there is no problem with combining.
As for shortening the prayer, then it is Sunnah even if he is residing. He prays two Rakats if he residing is four days or less. However, if he is determined upon residing more than four days, then he prays four according to the majority of the people of knowledge. As for if he doesn’t know when he will travel and he doesn’t have determination [wherein] he may stay two days or three or more [and] he isn’t determined, than this one shortens his prayer even if the time period is long. He prays two Rakaats and he can combine even if the time is long as long as he doesn’t have certainty regarding when he will travel. [This is] because he is still waiting [to complete] his need. [If] he doesn’t know when he will attain it, then this one’s ruling [i.e. this person’s ruling] is that of travelling….
Majid Jawed Al-Afghanee