Narrated Zaid bin Aslam from his father:
I was with Ibn 'Umar (radiallaahu `anhu) on the way to Mecca, and he got the news that Safiya bint Abu Ubaid was seriously ill. So, he hastened his pace, and when the twilight disappeared, he dismounted and offered the Maghrib and 'Isha' prayers together. Then he said, "I saw that whenever the Prophet (sallallaahu `alayhi wasallam) had to hasten when traveling, he would delay the Maghrib prayer and join them together (i.e. offer the Maghrib and the Isha prayers together)."
We know that prayers are shortened during travel only. But what about combining prayers? Does a person have to be travelling in order to combine two prayers, or is it allowed otherwise as well? Here's a general answer from IslamQA:
It is not permissible for a Muslim man or woman to delay an obligatory prayer beyond the proper time, rather every accountable Muslim man and woman is obliged to perform the prayers on time as much as they can.
Work is not an excuse for delaying prayer, neither is impurity on clothes or clothes being dirty. None of these are acceptable excuses.
People should be excused from work at prayer times; at the time of prayer a worker has to wash the impurity from his clothes, or change into clean clothes. As for regular dirt (as opposed to impurities), this does not prevent one from praying in those clothes, so long as that dirt is not impure and does not have an offensive smell that would bother other worshippers. But if the dirt or its smell will bother others, then he has to wash it before praying, or change into clean clothes so that he can pray in congregation.
It is permissible for those who have legitimate shar’i excuses, such as those who are sick or are travelling, to combine Zuhr and ‘Asr at the time of either of them, and to combine Maghrib and ‘Isha’ at the time of either of them.
This was narrated in the saheeh Sunnah of the Prophet (peace and blessings of Allaah be upon him). It is also permissible to join prayers when there is heavy rain or mud, which cause difficulty for people.
P.S. given below is a 'general knowledge' question for you.. Test yourself.