There are many reasons, any one of which are sufficient, to rule smoking as prohibited. Most importantly, it is harmful to the Deen, health, environment, family, brotherhood and social relations, property, etc.
Smoking damages a person’s act of worship, and reduces their rewards. For instance, it ruins the salaah, which is a pillar of Deen. The Prophet e said: ‘Whoever eats garlic or onion, let him avoid us and our masjid, and stay in his home. The angels are surely hurt by things that hurt the human beings.’ (Al-Bukhari and Muslim) ( i am not quite sure of this one 
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Those people, with clean and pure nature, have no doubt that the smell emanating from the mouth of a smoker is worse and more foul than that from the mouth of one who ate garlic or onion. Therefore, a smoker has two options, either to harm the praying people and the angels with his foul smell, or abstain from the prayer with jama’ah.
Smoking also ruins fasting. Fasting becomes very hard on the smoker. As soon as the day is over, he hastens to break his fast on an evil cigarette instead of sweet dates or pure water. Even if he fasts throughout the month of Ramadhaan he will be reluctant to fast on other days. Thus he loses the great reward of those who fast even one day in Allah’s path.
The harm that smoking does to the human body is undeniable. The medical evidence for this is well established and overwhelming.
Smoking contains poisonous materials, such as nicotine, tar, carbon monoxide, arsenic, benzopyrene, etc., which the smoker inhales in small proportions. Their harm accumulates in time, resulting in a gradual damage of the human organs and tissues.
The hazards of smoking to health are hard to count. Cancer, tuberculosis, heart attacks, asthma, coughing, premature birth, infertility, infections in the digestive system, high blood pressure, nervousness, mouth and teeth diseases, etc., are among the many health hazards that have been strongly linked to smoking.
These diseases may not all appear at once, but a smoker runs the risk of suffering from some of them, and his suffering increases as he grows older. Furthermore, statistics have established that smokers, on the average, reduce their age by ten years.
This in itself is sufficient to prohibit smoking. Islam prohibits any action that causes harm to oneself or to other people. Allah Ta’ala says :
‘Do not kill yourselves, Allah is indeed merciful to you.’
(An-Nisa’ 4-29)
‘Do not cast yourselves, with your own hands, into destruction.’
(Al-Baqarah 2:195)
And the Messenger sallallahu alaihe wasallam says: ‘No harm may be inflicted on oneself or others.’
(Ahmad and Ibn Maajah from Ibn Abbaas and Ubaadah)
‘The feet of a human being will not depart, on the day of Judgement, from his standing before his Lord, until he is questioned about five things: his lifetime - how did he pass it, his youth - how did he used it, his wealth - where did he earn it and how did he spend it, and how did he follow what he knew.’
(At-Tirmithi and others from Ibn Mas‘ud and Abu Barzah )
‘Whoever consumes poison, killing himself with it, then he will he consuming his poison in the hellfire, and he will abide in it permanently and eternally.’
(Al-Bukhari and Muslim from Jaabir)
Smoking is also harmful to the human mind and its power to reason. An obvious demonstration of this is that one who is addicted to it passes through periods of severe craving, making it hard for him to think, concentrate, solve a problem, or do any important matter, until he smokes.
His digestive system is also affected, causing him frequent nervousness and trembling of the hands. He passes through periods of excitability and he becomes an insomniac.
A smoker, whether he likes it or not, makes an example of himself for his children and others to follow. He leads them to commit this evil. Actions sometimes have a stronger effect than words. Thus, even if he advises them or forbids them from smoking, his partaking of it provides them with a strong excuse to do it
