Final answer:
The sentence that Hassan uses to show loyalty to Amir and that Amir uses at the end of the novel as a promise to Sohrab is: "For you, a thousand times over."
Step-by-step explanation:
The sentence that Hassan uses to show loyalty to Amir and that Amir uses at the end of the novel as a promise to Sohrab is: "For you, a thousand times over." This sentence is significant in the novel and represents the deep bond between Hassan and Amir.
It reflects Hassan's devotion to Amir and his willingness to sacrifice himself for him.