What is the view of Imam Ahmad, and of the other imams, on the prayer of a captive, whether resident or traveling: does he pray in full or shorten?

There is no difference between a resident captive and other residents regarding the obligation to complete the prayer. As for the traveling captive, the Hanbalis, Hanafis, and Malikis shorten the prayer if their captor travels with them a distance that permits shortening. The Shafi'is hold that if a captive is taken by disbelievers and does not know their destination, he does not shorten the prayer until after two days. If he knows the destination and intends to escape, he does not shorten it before two stages. However, if he intends to go to a country without committing a sin, he shortens the prayer immediately if the distance is one that permits shortening.