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.
Intoxicants and drugs, including hashish, are prohibited based on the indication of contiguous Sharee'ah texts. More than one of the companions, including Jaabir and Abu Hurayrah narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: “Every intoxicant is prohibited.” [Al-Bukhari, Muslim, An-Nasaa’i, and others] In another narration, he, sallallaahu ‘alayhi wa sallam, said: “…and I forbid you from every intoxicant.”
The term 'intoxicant' refers to every substance that obscures the reason of whoever consumes it and causes a disturbance of the mind. Accordingly, it is incumbent upon you to advise this man first, if possible, and explain the danger of his act to him. If he does not desist, or you are not able to advise because of a potential harm he may cause to you as a reaction to your advice, then you should hasten to inform the anti-drug authorities and never be lax towards this issue. This is because what this man is doing leads to evil consequences with regard to the individual and the society as well.
Allaah Knows best.