Hazrat Umar says that combining two prayers without a valid excuse is among the major sins.

Hazrat Abdur Rahman bin Muheb says that I came to Hazrat Salim and asked him, "O Abu Umar! Do you combine two prayers while traveling?" He replied, "No, but if I am in a hurry to travel, then I do."

It was mentioned before Hazrat Muhammad bin Sirin that Hazrat Jabir bin Zaid used to combine two prayers. He said that he must have combined two prayers for some reason.

Hasan and Muhammad say that in our opinion, combining two prayers during travel or at home is not part of the religion. However, at Arafat, Zuhr and Asr prayers will be combined, and at Muzdalifah, Maghrib and Isha prayers will be combined.

Hazrat Abdur Rahman bin Harmalah says that a man came to Hazrat Saeed bin Musayyib and said, "I am a shepherd of camels. I search for them and when evening comes, I perform the Maghrib prayer. Then I give rest to myself and then I fall asleep before the Isha prayer." He (Saeed bin Musayyib) said, "Do not sleep before performing the Isha prayer. If you fear sleep, then combine both prayers."

Hazrat Ata and Hazrat Dhahak say that if a sick person wishes, he can combine two prayers.

8479 - From Juwaybir, from al-Dahhak regarding the sick person who prays, they said: If he wishes, he may combine the two prayers.

Hazrat Ata says that a shepherd will perform the prayer of a traveler.

Hazrat Saeed bin Jubair and Hazrat Abu Al-Bakhtari say that when swords are clashing and people are killing each other, and the time for prayer arrives, then you should say: Subhan Allah, Alhamdulillah, La ilaha illallah, Allahu Akbar. This is your prayer; then do not repeat the prayer.

Hazrat Mujahid and Hazrat Hakam state that when horsemen are engaged with each other and swords are clashing, then at the time of prayer, it is sufficient for a person to say the takbeer. If he says even one takbeer facing any direction, it is sufficient for him.