I know that cigarette smoking is haraam in Islam, but why is it so? Please give all the reasons from Qur'an and sunnah.

All perfect praise be to Allaah, The Lord of the Worlds.  I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Smoking is Haraam (prohibited) due to its many harmful effects and evil consequences. This is a well-known and universally accepted fact. No one could seriously doubt or refute this truth. The harms of smoking have been proved through evidence that doctors have agreed upon. The tobacco that is smoked is something evil and Almighty Allaah has made lawful for his slaves everything that is good and beneficial and has forbidden all that is evil and harmful. Allaah Says (what means): {They ask you (O Muhammad) what has been made lawful for them. Say, “Lawful for you are At-Tayyibaat [foods which are lawful]…}[Quran 5:4]

Almighty Allaah states the very purpose for which He sent the Prophet, sallallaahu ‘alayhi wa sallam , Saying (what means): {He allows them that which is lawful (At-Tayibaat) and prohibits them that which is unlawful (Al-Khabaa’ith).}[Quran 7:157]

Therefore and in brief, smoking is a means of killing oneself slowly and Almighty Allaah has forbidden killing oneself, so no one should differ in this prohibition (i.e. prohibiting smoking).

Allaah Knows best.