Provide an instance when Amir was cruel to Hassan?

I assume by your other questions that you are referring to chapter 3-4. Amir can treat Hassan badly at times. Amir lets his education and social position go to his head. Hassan is illiterate and Amir will sometimes tease him with words he does not know. At one point Amir plays with Hassan using the word "imbecile." Amir will come to regret this later in life.