is smoking a sin or is it not just good?

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Yesterday, I was in the company of my friends. we were four in number. One of us smokes a lot. He is a muslim. I don't smoke and so also the other two guys. the other two guys are christian and so also myself. While we were together and discussing on general topics, the friends that smoke brought out his cigarette and his lighter just have a shot. Then one of the other two friends retorted sharply and said "smoking is sinful". Then I asked if he means whether smoking is bad or is it against the law of God in either Islam or Christianity? He answered me that smoking is not only bad but sinful. Then i asked him why? he only told me it is sinful because it imoral. I seem not to agree with him. pls give example and references where applicable.