Who did Jesus say had sinned to cause the man in John 9 to be born blind?

Answer: Neither he nor his parents

Asked whether the man or his parents sinned, Jesus said neither; this happened so that the works of God would be displayed in him.

“Now as Jesus was passing by, He saw a man blind from birth, and His disciples asked Him, “Rabbi, who sinned, this man or his parents, that he was born blind?” Jesus answered, “Neither this man nor his parents sinned, but this happened so that the works of God would be displayed in him.”— John 9:1-3 (BSB)