In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Smoking is forbidden due to the evidences found in the Qur’aan and the Sunnah, and based on sound opinion. Allaah [The Mighty and Majestic] said:
[وَلا تَقْتُلُوا أَنْفُسَكُمْ إِنَّ اللَّهَ كَانَ بِكُمْ رَحِيماً – And do not kill yourselves (nor kill one another). Surely, Allah is Most Merciful to you] [Soorah Nisaa: Ayah: 29]
It is well-known that smoking causes harm to the body which may appear sooner or later, and this is an affair agreed upon by the doctors of today after the advancement of medicine. Smoking also disfigures the teeth, the lips etc.
Also, Allaah [the Most High] said: [وَلا تُؤْتُوا السُّفَهَاءَ أَمْوَالَكُمُ الَّتِي جَعَلَ اللَّهُ لَكُمْ قِيَاماً – And give not unto the foolish your property which Allah has made a means of support for you]. Therefore, Allaah clarified the wisdom behind wealth and that He made it a means of support through which the Religious and worldly benefits are established. And it is well known that spending wealth on cigarettes neither supports Religious nor worldly benefits, rather it does the opposite.
And from the evidences in the Sunnah concerning the impermissibility of smoking is that the Prophet [sallal-laahu-alayhi-wasallam] forbade the squandering of wealth, and squandering wealth is when it is spent in other than what is beneficial. It is well known that spending wealth on smoking is a spending in which there is no benefit, rather it is spending in that which is harmful.
As for sound opinion, then sound intellect necessitates that a person does not partake in that which harms him and wastes his wealth, whilst he is a believer and that he will be brought to account. Therefore, sound intellect necessitates that a person does what benefits him and leaves what harms him, and from the perfection of [one’s religion and character] is to leave that which is of no benefit.
The affairs are three:  Beneficial things  Harmful things  Things in which there is neither harm nor benefit. The first affair is sought after, the second affair is blameworthy, and regarding the third affair, a person refrains from it out of seeking perfection in his affairs, but there is no blame on him if he does it.
[Paraphrased and abridged: Source: links http://www.ajurry.com/vb/showthread.php?t=8074] http://www.ibnothaimeen.com/all/noor/article_3610.shtml