[The ruling on smoking in the mosque]

Answer:
 Smoking, in itself, is permissible and not prohibited, due to the absence of a proof indicating prohibition.
 If smoking in the mosque leads to harming the people of the mosque—or some of them—then it is not permissible.
 If smoking in the mosque does not cause harm to anyone in the mosque, then there is no sin upon the smoker in it.
 If smoking in the mosque leads to people’s denunciation, reviling the smoker, blaming him, and tearing away his cover, then it is not permissible—for a believer must guard his reputation. It has been related from ʿAlī (peace be upon him): “Leave what people denounce—even if you have your excuse for it.”
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1