Combining two prayers before travelling
It is permissible for the person who is not travelling to combine two prayers if it will be too difficult for him to perform the second prayer on time. Permission to join prayers is broader than permission to shorten them, because shortening them is allowed only to travelers, but both travelers and non-travellers are permitted to join prayers when it will be difficult to perform the second prayer, whether on time or in congregation.
Based on this, it is permissible for you to bring ‘Asr forward and pray it with Zuhr in your home city, if you think it most likely that you will not be able to pray it on time because of travelling.
If the airport is outside your city, you can pray in the airport and shorten your prayers. If you live near the airport, then you can join the prayers but you cannot shorten them.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said:
If a person is in Qaseem and goes out to the airport, can he shorten his prayers in the airport?
Yes, he may shorten his prayers, because he has left the built-up area of his town, and all the villages that are around the airport are separate from the town. But if a person lives near the airport, he should not shorten his prayer, because he has not yet left the built-up area of his town.
Al-Sharh al-Mumti’, 4/514
It was narrated that Ibn ‘Abbaas said: The Messenger of Allaah (peace and blessings of Allaah be upon him) prayed Zuhr and ‘Asr together in Madeenah, at a time when there was no fear and he was not travelling. Abu’l-Zubayr said: I asked Sa’eed [i.e., ibn Jubayr], Why did he do that? He said, I asked Ibn ‘Abbaas the same question, and he said, He did not want anyone among his ummah to suffer hardship.
Narrated by Muslim, 705.
Shaykh al-Islam Ibn Taymiyah said:
The specific reason for shortening the prayers is travelling, and it is not permissible to shorten prayers when not travelling. With regard to joining prayers, the reason for it is if there is a need or an excuse. So if a person needs to do so, he may join prayers whilst travelling, whether he is travelling a long distance or a short one, and he may join prayers because of rain etc., or because of sickness etc., or for any other reason, because the point is to spare the Muslims any difficulty or hardship.
Majmoo’ al-Fataawa, 22/293; published by King Fahd Complex.