Musannaf Ibn Abi Shaybah

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.

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